"The subjects treated of in the Epistle are many and various; moreover, St. James not infrequently, whilst elucidating a certain point, passes abruptly to another, and presently resumes once more his former argument; hence it is difficult to give a precise division of the Epistle."
The object of the writer was to enforce the practical duties of the Christian life. The vices against which he warns them are: formalism, which made the service of God consist in washings and outward ceremonies, whereas he reminds them (1:27) that it consists rather in active love and purity; fanaticism, which, under the cloak of religious zeal, was tearing Jerusalem in pieces (1:20); fatalism, which threw its sins on God (1:13); meanness, which crouched before the rich (2:2); falsehood, which had made words and oaths play-things (3:2-12); partisanship (3:14); evil speaking (4:11); boasting (4:16); oppression (5:4). The great lesson which he teaches them as Christians is patience, patience in trial (1:2), patience in good works (1:22-25), patience under provocation (3:17), patience under oppression (5:7), patience under persecution (5:10); and the ground of their patience is that the coming of the Lord drawing nigh, which is to right all wrong (5:8).
The author identifies himself in the opening verse as "James, a servant of God and of the Lord Jesus Christ". From the middle of the third century, patristic authors cited the Epistle as written by James the Just, a relation of Jesus and first Bishop of Jerusalem. Not numbered among the Twelve Apostles, unless he is identified as James the Less, James was nonetheless a very important figure: Paul described him as "the brother of the Lord" in Galatians 1:19 and as one of the three "pillars of the Church" in 2:9. He is traditionally considered the first of the Seventy Disciples. John Calvin and others suggested that the author was the Apostle James, son of Alphaeus, who was often identified with James the Just. If written by James the Just, the place and time of the writing of the epistle would be Jerusalem, where James was residing before his martyrdom in 62.
Authorship has also occasionally been attributed to the apostle James the Great, brother of John the Evangelist and son of Zebedee. The letter does mention persecutions in the present tense (2:6), and this is consistent with the persecution in Jerusalem during which James the Great was martyred (Acts 12:1). However, some challenge the early date on the basis of some of the letter’s content, which they interpret to be a clarification of St. Paul’s teachings on justification found in his Epistle to the Romans, written c. 54. If written by James the Great, the location would have also been Jerusalem, sometime before 45.
Lastly, many scholars consider the epistle to be written in the late first or early second centuries, after the death of James the Just. Among the reasons for this are:
the author introduces himself merely as "a servant of God and of the Lord Jesus Christ", without invoking any special family relationship to Jesus.
the cultured Greek language of the Epistle, it is contended, could not have been written by a Jerusalemite Jew. This argument has lost much force as recent insight into Greek influence on Judea at the time has come to light. It is plausible that the letter in Greek to the Jewish diaspora could have been composed with a secretary, as Jerome argued. Some scholars argue for a primitive version of the letter composed by James and then later polished by another writer.
the author fails to mention Jewish ritual requirements such as circumcision, whereas James the Just is known from Galatians and the Acts of the Apostles to have been particularly concerned with ministering to the Jewish and circumcised (however, since it is addressed to a Jewish audience, such requirements would naturally be taken for granted; moreover, the Epistle could have been written before the end of Paul's First Missionary Journey (AD 46-48), when the inclusion of gentiles first became an issue).
the author fails to mention any details of Jesus's life (however, the doctrines resemble Jesus's own doctrines as recorded in the Gospels, more than Paul's doctrines).
the epistle was only gradually accepted into the (non-Jewish) canon of the New Testament.
Some see parallels between James and 1 Peter, 1 Clement, and the Shepherd of Hermas and take this to reflect the socio-economic situation Christians were dealing with in the late first or early second century. It thus could have been written anywhere in the Empire where Christians spoke Greek. There are some scholars who argued for Syria.
The Interpreter's Bible calls James "... a Christian revision of a Jewish work.” TIB XII p. 21 James’ epistle is so Jewish that Adam Clarke cites Talmudic sources for nearly every verse. The Interpreters’ Bible posits a preexistent Jewish “Book of Jacob” adapted to a Christian audience; rather than point out Jewish antecedents, it highlights the less numerous Christian accretions.
“In the last decade of the nineteenth century … a French scholar” [L. Massebieau] “and a German scholar,” [Friedrich Spitta] “working wholly independently, published almost simultaneously conclusions that were identical. Both maintained that the epistle was originally a purely Jewish writing which has been converted into a Christian work by an editor who merely added ‘and of the Lord Jesus Christ’ in 1:1 and ‘our Lord Jesus Christ’ in 2:1. Both writers stressed in support of their theory the extraordinarily difficult grammatical problem offered by the Greek genitives in 2:1 … a problem solved at once by the theory of the interpolation. And they argued further that if this interpolation is accepted, a corresponding interpolation in 1:1 may be inferred; especially since 1:1, as it now reads, contains language unique in the New Testament… Then, since these two occurrences of ‘Jesus Christ’ are the only explicit Christian terms in the letter, the remainder, they argued, not only represented a use of Jewish tradition, but was Jewish tradition and nothing else.
“… a generation later Arnold Meyer … [n]oting that in Greek, Aramaic, and Hebrew, ‘James’ and ‘Jacob’ are the same word,… saw that if the Christian ‘interpolation’ in 1:1 was recognized as such, the original opening words could be read “Jacob, a servant of God, to the twelve tribes in the dispersion: Greeting.’… And for a letter from Jacob to the ‘twelve tribes’ a well-known biblical precedent was provided by Gen.” [Genesis] “49, where Jacob addresses the ‘ancestor’ of each tribe in turn…. Meyer undertook to demonstrate that similar references to the twelve tribes can be detected in James
“But even if Meyer is correct in his contention that a ‘Letter of Jacob’ forms the basis of James, it by no means follows that he is equally correct in contending that the former can be recovered by eliminating minimal Christian additions in 1:1; 2:1; 5:12; and 5:14. He seems vastly to have underestimated the contributions of the Christian editor. This appears most vividly in the long section 2:14-26 on the relative value of faith and works. … Not only is the general trend of the argument in 2:14-26 one impossible in Judaism, but the details of its wording show that the argument is directed against a non-Jewish opponent – an opponent who can be identified definitely as Paul… Only one conclusion appears to be possible: 2:14-26 was written not by a Jew, but by a Christian.
“Nor is 2:14-26 the only Christian passage in James.
“If this is correct, we have the solution of a difficulty in Meyer’s theory for which he has no satisfying answer." 
James, as is also the case with “certain other epistles, toward the close of some Pauline letters... and in Heb.” [Hebrews] “13” [has] “sequences of sayings-groups and isolated sayings arranged with little apparent logical order… this form was very familiar in the contemporary Hellenistic world, where similar somewhat miscellaneous collections of general moral instructions were widely employed in teaching ethics. If these instructions were phrased throughout in the third person such a collection was called a ‘gnomologium.’ But if the second person (singular or plural) was employed, so that the teachings were addressed – either actually or as a literary device – to an individual or to a group, then the collection was termed a ‘paraenesis.’ And in James we have a perfect example of a paraenesis…
“While James is a paraenesis, as a whole and in all its parts, in many sections another highly specialized contemporary literary form is also evident – the form known as the ‘diatribe.’… for the present purposes it may be described adequately enough as copying the style of a speaker engaged in a lively oral debase with an opponent. Among ancient writers on ethics who use the diatribe form Epictetus is particularly notable; among Jewish authors the thoroughly Hellenized Philo often employs it. In the New Testament, Rom.” [Romans] “3:1-8 illustrates the form admirably.
“The fact that the Epistle of James is written throughout as a paraenesis, with frequent employment of the diatribe, shows that its author must be sought among those whose literary associates were with the Greek rather than with the Hebrew world. For the antecedents of true prose paraenesis among non-Greek-speaking Jews are so scanty as to be virtually nonexistent.
“On the other hand, the content of James, as contrasted with its literary form, belongs unequivocally to the Hebraic-Christian, and not to the Hellenistic world…. James, as we have it, is unambiguously the work of a Christian author, whose training was Hellenistic but whose religious background was firmly Hebraic." TIB XII 1955
According to Sophie Laws' (revised by Walter T. Wilson) introduction to the Letter of James in the Harper CollinsNRSVStudy Bible, "The text opens with typical epistolary greetings but has no comparable ending, indicating that it may be a letter in literary form only, not a real piece of correspondence...The text has often been described as Christian wisdom literature, because like Proverbs and Sirach, it consists largely of moral exhortations and precepts of a traditional and eclectic nature." 
The Epistle of James was included among the 27 New Testament books first listed by Athanasius of Alexandria and was confirmed as a canonical epistle of the New Testament by a series of councils in the fourth century. Today, virtually all denominations of Christianity consider this book to be a canonical epistle of the New Testament. See Biblical canon
In the first centuries of the Church the authenticity of the Epistle was doubted by some, and among others by Theodore, Bishop of Mopsuestia in Cilicia; it is therefore deuterocanonical. It is missing in the Muratorian fragment, and because of the silence of several of the western churches regarding it, Eusebius classes it among the Antilegomena or contested writings (Historia ecclesiae, 3.25; 2.23). St. Jerome gives a similar appraisal but adds that with time it had been universally admitted. Gaius Marius Victorinus, in his commentary on the Epistle to the Galatians, openly questioned whether the teachings of James were heretical.
Its late recognition in the Church, especially in the West, may be explained by the fact that it was written for or by Jewish Christians, and therefore not widely circulated among the Gentile Churches. There is some indication that a few groups distrusted the book because of its doctrine. In Reformation times a few theologians, most notably Martin Luther, argued that this epistle was too defective to be part of the canonical New Testament. This is probably due to the book's specific teaching that faith alone is not enough for salvation (James 2:24), which seemed to contradict Luther's doctrine of sola fide (faith alone).
The letter contains the following famous passage concerning salvation and justification:
“What does it profit, my brethren, if someone says he has faith but does not have works? Can such faith save him? …You see then that a man is justified by works, and not by faith only…? For as the body without the spirit is dead, so faith without works is dead also.” (James 2:14, 24, 26)
This passage has been cited in Christian theological debates, especially against the Protestant doctrine of Justification by faith alone. Gaius Marius Victorinus (4th century) associated James's teaching on works with the heretical Symmachian sect, followers of Symmachus the Ebionite, and openly questioned whether James's teachings were heretical. This passage has also been contrasted with the teachings of Paul of Tarsus, especially in his Epistle to the Romans (see Romans 3:28). One issue in the debate is the proper rendering of the Greek δικαιωθηναι (dikaiōthēnai). But see also New Perspective on Paul.
"Are any among you sick? They should call for the elders of the church and have them pray over them, anointing them with oil in the name of the Lord. And their prayer offered in faith will heal the sick, and the Lord will make them well. And anyone who has committed sins will be forgiven." (5:14,15).
↑Catechism of the Catholic Church at vatican.va: "1815 The gift of faith remains in one who has not sinned against it. But "faith apart from works is dead":[Jas 2:26] when it is deprived of hope and love, faith does not fully unite the believer to Christ and does not make him a living member of his Body."
↑"History of the Christian Church, book 7, chapter 4". http://www.ccel.org/s/schaff/history/7_ch04.htm.; Philip Schaff's The Protestant Spirit of Luther’s Version: "The most important example of dogmatic influence in Luther’s version is the famous interpolation of the word alone in Rom. 3:28 (allein durch den Glauben), by which he intended to emphasize his solifidian doctrine of justification, on the plea that the German idiom required the insertion for the sake of clearness. But he thereby brought Paul into direct verbal conflict with James, who says (James 2:24), "by works a man is justified, and not only by faith" ("nicht durch den Glauben allein"). It is well known that Luther deemed it impossible to harmonize the two apostles in this article, and characterized the Epistle of James as an "epistle of straw," because it had no evangelical character ("keine evangelische Art")."
↑Catholic Encyclopedia: The Brethern of the Lord: "His [James the brother of the Lord] identity with James the Less (Mark 15:40) and the Apostle James, the son of Alpheus (Matthew 10:3; Mark 3:18), although contested by many Protestant critics, may also be considered as certain."
↑Oxford Bible Commentary p.1256, Oxford University Press 2001
↑ Oxford Bible Commentary p.1256, Oxford University Press 2001
↑The New Testament of our Lord and Saviour Jesus Christ. The text carefully printed from the
most correct copies of the present Authorized Version. Including the marginal readings and parallel texts. With a Commentary and Critical Notes. Designed as a help to a better understanding of the sacred writings. By Adam Clarke, LL.D. F.S.A. M.R.I.A. With a complete alphabetical index. Royal Octavo Stereotype Edition. Vol. II. [Volume VI together with the
Old Testament volumes in Dad’s set] New York, Published by J. Emory and B. Waugh, for the
Methodist Episcopal Church, at the conference office, 13 Crosby-Street. J. Collord, Printer.
↑TIB XII pp. 4-11 TIB XII is abbreviation for The Interpreters Bible The Holy Scriptures in the King James and Revised Standard versions
with general articles and introduction, exegesis, [and] exposition for each book of the Bible in
twelve volumes, George Arthur Buttrick, Commentary Editor, Walter Russell Bowie, Associate
Editor of Exposition, Paul Scherer, Associate Editor of Exposition, John Knox Associate Editor
of New Testament Introduction and Exegesis, Samuel Terrien, Associate Editor of Old
Testament Introduction and Exegesis, Nolan B. Harmon Editor, Abingdon Press, copyright 1955
by Pierce and Washabaugh, set up printed, and bound by the Parthenon Press, at Nashville,
Tennessee, Volume XII, The Epistle of James [Introduction and Exegesis – Burton Scott Easton, Exposition - Gordon Poteat], the First and Second Epistles of Peter, The first, Second, and Third Epistles of John, The Epistle of Jude, The Revelation of St. John the Divine, General Articles, Indexes
↑ Sophie Laws (1993). "The Letter of James". in Wayne A. Meeks et al.. The HarperCollins Study Bible: New Revised Standard Version, with the Apocryphal/Deuterocanonical Books. New York: HarperCollins, p. 2052
↑ Grant, Robert M. The Formation of the New Testament. New York: Harper & Row, 1965.p. 155, there are two possible allusions to James in Adversus Haereses. They are in 4.16.2 (James 2:23) and 5.1.1 (James 1:18,22)
The most important example of dogmatic influence in Luther’s version is the famous interpolation of the word alone in Rom. 3:28 (allein durch den Glauben), by which he intended to emphasize his solifidian doctrine of justification, on the plea that the German idiom required the insertion for the sake of clearness. But he thereby brought Paul into direct verbal conflict with James, who says (James 2:24), "by works a man is justified, and not only by faith" ("nicht durch den Glauben allein"). It is well known that Luther deemed it impossible to harmonize the two apostles in this article, and characterized the Epistle of James as an "epistle of straw," because it had no evangelical character ("keine evangelische Art").