Sola scriptura (Latinablative, "by scripture alone") is the doctrine that the Bible is the only infallible or inerrant authority for Christian faith, and that it contains all knowledge necessary for salvation and holiness. Consequently, Sola Scriptura demands that no doctrine is to be admitted or confessed that is not found directly or logically within Scripture. However, Sola Scriptura is not a denial of other authorities governing Christian life and devotion. Rather, it simply demands that all other authorities are subordinate to, and are to be corrected by, the written word of God. Sola scriptura was a foundational doctrinal principle of the Protestant Reformation held by the Reformers and is a formal principle of Protestantism today (see Five solas).
During the Reformation, authentication of Scripture was governed by the discernable excellence of the text as well as the personal witness of the Holy Spirit to the heart of each man. Furthermore, per Sola Scriptura, the relationship of Scriptural authority to pastoral care was well exampled by the Westminster Confession of Faith which stated:
VII. All things in Scripture are not alike plain in themselves, nor alike clear unto all; yet those things which are necessary to be known, believed, and observed, for salvation, are so clearly propounded and opened in some place of Scripture or other, that not only the learned, but the unlearned, in a due use of the ordinary means, may attain unto a sufficient understanding of them.
Here the phrase "due use of the ordinary means" includes appeals to pastors and teachers
(Ephesians 4:11-14). As such, Sola Scriptura reflects a careful tension between the perspicuity (clarity) of Scripture necessary for its role as final authority, and the occasional need for its meaning to be revealed by exposition (Hebrews 5:12).
Beyond the Reformation, as in some Evangelical and Baptist denominations, Sola Scriptura is stated even more strongly: it is self-authenticating, clear (perspicuous) to the rational reader, its own interpreter ("Scripture interprets Scripture"), and sufficient of itself to be the final authority of Christiandoctrine.
Sola scriptura is one of the five solas, considered by some Protestant groups to be the theological pillars of the Reformation. The key implication of the principle is that interpretations and applications of the Scriptures do not have the same authority as the Scriptures themselves; hence, the ecclesiastical authority is viewed as subject to correction by the Scriptures, even by an individual member of the Church.
Luther said, "a simple layman armed with Scripture is greater than the mightiest pope without it". The intention of the Reformation was to correct the perceived errors of the Catholic Church by appeal to the uniqueness of the Bible's authority and to reject what Catholics considered to be Apostolic Tradition as a source of original authority alongside of the Bible, wherever Tradition did not have biblical support or where it supposedly contradicted Scripture.
Sola scriptura, however, does not ignore Christian history and tradition when seeking to understand the Bible. Rather, it sees the Bible as the only final authority in matters of faith and practice. As Martin Luther said, "The true rule is this: God's Word shall establish articles of faith, and no one else, not even an angel can do so."
The term heretical is commonly used by Protestants who denounce teachings and institutions that they accordingly view as deviating from Scripture.
Lutherans believe that the Bible, as a divinely inspired book, is the source of all revealed divine knowledge. Scripture alone is the formal principle of the faith, the final authority for all matters of faith and morals because of its inspiration, authority, clarity, effectiveness, and sufficiency.
The Bible does not merely contain the Word of God, but every word of it is, because of verbal inspiration, the direct, immediate word of God. As Lutherans confess in the Nicene Creed, the Holy Spirit "spoke through the prophets". The Apology of the Augsburg Confession identifies Holy Scripture with the Word of God and calls the Holy Spirit the author of the Bible. Because of this, Lutherans confess in the Formula of Concord, "we receive and embrace with our whole heart the prophetic and apostolic Scriptures of the Old and New Testaments as the pure, clear fountain of Israel." The apocryphal books were not written by the prophets, by inspiration; they contain errors were never included in the Palestinian Canon that Jesus used, and therefore are not a part of Holy Scripture. The prophetic and apostolic Scriptures are authentic as written by the prophets and apostles. A correct translation of their writings is God's Word because it has the same meaning as the original Hebrew and Greek. A mistranslation is not God's word, and no human authority can invest it with divine authority.
Holy Scripture, the Word of God, carries the full authority of God. Every single statement of the Bible calls for instant and unqualified acceptance. Every doctrine of the Bible is the teaching of God and therefore requires full agreement. Every promise of the Bible calls for unshakable trust in its fulfillment. Every command of the Bible is the directive of God himself and therefore demands willing observance.
The Bible presents all doctrines and commands of the Christian faith clearly. God's Word is freely accessible to every reader or hearer of ordinary intelligence, without requiring any special education. Of course, one must understand the language God's Word is presented in, and not be so preoccupied by contrary thoughts so as to prevent understanding. As a result of this, no one needs to wait for any clergy, and pope, scholar, or ecumenical council to explain the real meaning of any part of the Bible.
Scripture is united with the power of the Holy Spirit and with it, not only demands, but also creates the acceptance of its teaching. This teaching produces faith and obedience. Holy Scripture is not a dead letter, but rather, the power of the Holy Spirit is inherent in it. Scripture does not compel a mere intellectual assent to its doctrine, resting on logical argumentation, but rather it creates the living agreement of faith. As the Smalcald Articles affirm, "in those things which concern the spoken, outward Word, we must firmly hold that God grants His Spirit or grace to no one, except through or with the preceding outward Word."
The Bible contains everything that one needs to know in order to obtain salvation and to live a Christian life. There are no deficiencies in Scripture that need to be filled with by tradition, pronouncements of the Pope, new revelations, or present-day development of doctrine.
Sola scriptura may be contrasted with prima scriptura, which holds that, besides canonical scripture, there are other guides for what a believer should believe, and how he or she should live. Examples of this include the general revelation in creation, traditions, charismatic gifts, mystical insight, angelic visitations, conscience, common sense, the views of experts, the spirit of the times or something else. Prima scriptura suggests that ways of knowing or understanding God and his will, that do not originate from canonized scripture, are in a second place, perhaps helpful in interpreting that scripture, but testable by the canon and correctable by it, if they seem to contradict the scriptures.
Sola scriptura rejects any original infallible authority, other than the Bible. In this view, all secondary authority is derived from the authority of the Scriptures and is therefore subject to reform when compared to the teaching of the Bible. Church councils, preachers, biblical commentators, private revelation, or even a message allegedly from an angel or an apostle are not an original authority alongside the Bible in the sola scriptura approach. Even though most Protestants look at scripture alone and no other authority, some theologians say that the Bible itself teaches against sola scriptura. They believe that if a person believes in the whole Bible then that person cannot not believe in sola scriptura. These theologians believe that those following the concepts of sola scriptura have personally perverted the meaning of either the Bible or sola scriptura.
Singular authority of Scripture
The idea of the singular authority of Scripture is the motivation behind much of the Protestant effort to translate the Bible into vernacular languages and distribute it widely. Protestants generally believe each Christian should read the Bible for themselves and evaluate what they have been taught on the basis of it. Roman Catholicism and Eastern Orthodoxy, motivated by their belief that authoritative doctrine can also come from tradition, have been more active in translating them as well as the Bible into the vernacular languages, though this has not always been the case. Traditions of these non-Protestant churches include the Bible, patristic, conciliar, and liturgical texts. Even prior to the Protestant movement, hundreds of vernacular translations of the Bible and liturgical materials were translated throughout the preceding sixteen centuries. Some Bible translations such as the Geneva Bible included annotations and commentary that were anti-Roman Catholic. Before the Protestant Reformation, Latin was almost exclusively utilized but it was understood by only the most literate.
According to sola scriptura, the Church does not speak infallibly in its traditions, but only in Scripture. As John Wesley stated in the 18th century, "In all cases, the Church is to be judged by the Scripture, not the Scripture by the Church." For this reason, sola scriptura is called the formal cause or principle of the Reformation.
Protestants argue that the Scriptures are guaranteed to remain true to their divine source; and, thus, only insofar as the Church retains scriptural faith is it assured of God's favor. Following such an argument, if the Church were to fall away from faith through Scripture (a possibility which Roman Catholics deny but Protestants affirm), its authority would be negated. Therefore, the early Protestants targeted for elimination traditions and doctrines they believed were based on distortions of Scripture, or were contrary to the Bible, but which the Roman Catholic Church considered scripturally-based aspects of the Christian faith, such as transubstantiation, the doctrine of purgatory, the veneration of images or icons, and especially the doctrine that the Pope in Rome is the head of the Church on earth (Papal supremacy).
Scripture and Tradition
The Roman Catholic Church against which the Reformers directed these arguments did not see Scripture and the Sacred Tradition of the faith as different sources of authority, but that Scripture was handed down as part of tradition (see 2 The 2:15, 2 Tim 2:2). Accepted traditions were also perceived by the Roman Church as cohesive in nature. The proper interpretation of the Scriptures was seen as part of the faith of the Church, and seen indeed as the manner in which Biblical authority was upheld (see Acts 15:28-29). The meaning of Scripture was seen as proven from the faith universally held in the churches (see Phil 2:1, Acts 4:32), and the correctness of that universal faith was seen as proven from the Scriptures and apostolic tradition (see 2 The 2:15, 2 The 3:6, 1 Cor 11:2). The Biblical canon itself was thus viewed by Rome as part of the Church's tradition, as defined by its leadership and acknowledged by its laity.
However, this view of scripture and tradition was not universally accepted. Throughout the history of the Church, movements have arisen within the Church or alongside of it which have disputed the official interpretation of the Scriptures. The leaders of these movements were often labeled heretics and their doctrines were rejected. According to Irenaeus, the JudaisticEbionites charged less than one hundred years after the Apostles that the Christians overruled the authority of Scripture by failing to keep the Mosaic Law, see also Biblical law in Christianity. Later, Arius (250-336), once he had been made a presbyter in Alexandria, began arguing that the teaching concerning the deity of Christ was an invention of men not found in Scripture and not believed by the Early Christians. The Church held that when disagreements over Scripture arise, the correct interpretation of the Bible will be consistent with how the Church authorities have believed in the past (see 2 Tim 2:2, 2 The 2:15, 1 Cor 11:2), as revealed by the Ecumenical Councils, the writings of the Apostles of Jesus and Fathers of the Church, the decisions of the Bishops of Rome and similar sources of Tradition.
However, the Reformers believed some tradition to be very seriously in conflict with the Scriptures: especially, with regard to teaching about the Church itself, but also touching on basic principles of the Gospel. They believed that no matter how venerable the traditional source, traditional authority is always open to question by comparison to what the Scriptures say. The individual may be forced to rely on his understanding of Scripture even if the whole tradition were to speak against him. This, they said, had always been implicitly recognized in the Church, and remains a fail-safe against the corruption of the Church by human error and deceit. Corruptions had crept in, the Reformers said, which seriously undermined the legitimate authority of the Church, and Tradition had been perverted by wicked men.
Sola scriptura is a doctrine that is not, in the words of the Westminster Confession of Faith 1.6 "expressly set down in scripture". However, it is claimed that it passes the second test of being part of "the whole counsel of God" because it is "deduced from scripture" "by good and necessary consequence", citing passages such as Isaiah 8:20: "To the law and to the testimony: if they speak not according to this word, it is because there is no light in them.". Jesus is also typically understood by Protestants as expressly nullifying unscriptural traditions in the (Jewish) church, when he says, for example in Mark 7:13: "thus making void the word of God by your tradition that you have handed down. And many such things you do."
Mathison's Apology & Explanation on Sola Scriptura & Tradition
Keith Mathison, author of The Shape of Sola Scriptura (2003), uses a historical approach in explaining the history and formation of sola scriptura. Mathison (2003) classifies the formation of "Tradition" over the history of Christendom, asserting four categories:
Tradition 0: often defined as solo scriptura. This view of scripture can be defined as the use of only scripture and nothing else. This view prohibits the use of any other tradition outside of the scripture in any form or fashion, and often leads to subjectivism and individualism. 
Tradition I: also defined as sola scriptura. This term is defined as the Scriptures are the sole infallible authority, and interpretation of the Scriptures is done in and by the Church within the context of the regula fidei, or rule of faith. 
Tradition II: known as a two-source concept of tradition. This view proposes that Divine Revelation consist equally of both Scriptures as well as a secret oral tradition handed down through the ages. This is the typical Roman Catholic position dogmatized since the Reformation. 
Mathison (2003) explains the doctrine of sola Scriptura in his book asserting that there are four major tenets of sola scriptura:
It is the sole source of revelation. This means that the scriptures are perfect and complete, inerrant and infallible, and sufficient as our sole source of revelation. 
It is the sole infallible authority and norm. The Scriptures are inspired and are the very words of God, and therefore they are without error (inerrant) and incapable of being in error (infallible). They are also authoritative, meaning that they are binding on us as the Word of God. 
It is interpreted in and by the Church. The Scripture is truth, but the church is the custodian and pillar of truth. Because the church is the body of Christ, and is a spiritual entity, the church has the authority to interpret correctly the spiritual word of God. The church is given authority by Christ to teach and preach the Scriptures, and has authority to set normative doctrinal boundaries for the Scriptures and its members. 
It is interpreted in the context of the regula fidei, or the rule of faith. The regula fidei is a summary of the apostolic doctrines and provides the hermeneutical context for the Church to interpret Scripture. This tradition is incorporated in the creeds and confessions of the Church, which serve as a written summary of what the Church believes the Scriptures to say.
Roman Catholic position
Roman Catholics and Eastern Catholics, on the other hand, argue the belief in the Bible as the sole source of faith is unhistorical, illogical, and destructive of unity. The Roman Catholic Church does not deny the fact that Christ and the apostles founded the church by preaching and exacting faith in their doctrines. Those who received their word as revelation from God did so solely on their divine authority, and if in the time of the apostles, faith consisted in submitting to authorized teaching, Roman Catholics teach it does so now also since the foundation of the church is thought to be immovable.
The Roman Catholic position is that it is illogical to base faith upon the private interpretation of the Bible. They take faith to consist of submitting to authority in which the last word rests with the teacher, whereas with private interpretation, the last word rests with the reader. The Roman Catholic Church teaches that private judgment is fatal to the theological virtue of faith and causes divisions in the people of God.
Pope Pius XII revolutionized Catholic biblical approach in his encyclical Divino Afflante Spiritu, in which he promoted unlimited biblical research, biblical discussions and more bible oriented sermons. During Vatican II, the Catholic Church, while maintaining that the bible is a part of tradition, attempted to give additional weight to biblical basing of its teaching.
The Reformation proceeded in three general directions: the Lutheran exclusivists, the Reformed and the Anabaptists. The Lutherans aimed at establishing an evangelical consensus immediately, but the Reformed brought diverse groups into international association with one another on more liberal principles, which damaged hopes of union with the Lutherans. Meanwhile, the Anabaptists espoused an alternative view of history in which the true Church became hidden or lost through the Great Apostasy dating from Constantine. From that time forward fragmentation based on sola scriptura has predominated within Protestantism, although rare movements toward union have achieved success.