The New Testament apocrypha are a number of writings by early Christians that give accounts of Jesus and his teachings, the nature of God, or the teachings of his apostles and of their lives. These writings often have links with those books which are regarded as "canonical". Not every branch of the Christian church is in agreement as to which writings are to be regarded as "canonical" and which are "
The word "apocrypha" means "hidden writings" and comes from the Greek through Latin. The general term is usually applied to the books that were considered by the church as useful, but not divinely inspired. As such, to refer to Gnostic writings as "apocryphal" is misleading since they would not be classified in the same category by orthodox believers..
That some works are categorized as New Testament Apocrypha is indicative of the wide range of responses that were engendered in the interpretation of the message of Jesus of Nazareth. During the first several centuries of the transmission of that message, considerable debate turned on safeguarding its authenticity. Three key methods of addressing this survive to the present day: ordination, where groups authorize individuals as reliable teachers of the message; creeds, where groups define the boundaries of interpretation of the message; and canons, which list the primary documents certain groups believe contain the message originally taught by Jesus (in other words, the Bible). Many early books about Jesus were not included in the canons, and are now termed apocryphal. Some of them were vigorously suppressed and survive only as fragments. The earliest lists of authentic works of the New Testament were not quite the same as modern lists; for example the Book of Revelation was long regarded as unauthentic, while Shepherd of Hermas was considered genuine by many Christians, and appears in several ancient Bibles..
The works that presented themselves as "authentic" but did not obtain general acceptance from within the churches are called New Testament Apocrypha. These are not accepted as canonical by most mainstream Christian denominations; only the Ethiopian Orthodox Church recognizes the Shepherd of Hermas, 1 Clement, Acts of Paul, and several Old Testament books that most other denominations reject, but it should be noted that this church does not adhere to an explicit canon.
The SyriacPeshitta, used by all the various Syrian Churches, originally did not include 2 Peter, 2 John, 3 John, Jude and Revelation (and this canon of 22 books is the one cited by John Chrysostom (~347-407) and Theodoret (393-466) from the School of Antioch).
Western Syrians have added the remaining 5 books to their NT canons in modern times (such as the Lee Peshitta of 1823). Today, the official lectionaries followed by the Malankara Syrian Orthodox Church, with headquarters at Kottayam (India), and the Chaldean Syrian Church, also known as the Church of the East (Nestorian), with headquarters at Trichur (India), still present lessons from only the 22 books of the original Peshitta. 
The Armenian Apostolic church at times has included the Third Epistle to the Corinthians, but does not always list it with the other 27 canonical New Testament books. This Church did not accept the Revelation into its Bible until A.D. 1200.
The New Testament of the Coptic Bible, adopted by the Egyptian Church, includes the two Epistles of Clement.
Books which are objectively known not to have existed in antiquity, such as the medieval Gospel of Barnabas, are usually not considered part of the New Testament Apocrypha. Among these are also the Libellus de Nativitate Sanctae Mariae (also named as the Nativity of Mary) and Latin Infancy Gospel. The latter two did not exist in the antiquity, and seem to be based on the earlier Infancy Gospels..
The rarity of information about the childhood of Jesus in the canonical Gospels led to a hunger of early Christians for more detail about the early life of Jesus. This was supplied by a number of 2nd century and later texts, known as infancy gospels, none of which was accepted into the biblical canon, but the very number of their surviving manuscripts attests to their continued popularity.
Since these mostly survive as quotes scattered amongst critical commentaries by Pauline Christianity, some modern theories suggest that these may be variations on one another, although the quotations from the Gospel of the Ebionites appear more distinct than the others. It has also been suggested that the Gospel of the Hebrews may have been an earlier version of the Greek Gospel of Matthew.
Many alternate edited versions of other gospels existed during the period of early Christianity. Sometimes, those attributed to the text state elsewhere that their text is the earlier version, or that their text excises all the additions and distortions made by their opponents to the more recognised version of the text. The church fathers insist that these people are incorrect (and indeed heretical) in their assertions, but some modern scholars do not. It remains to be seen whether any are earlier and more accurate versions of the canonical texts. Details of their contents only survive in the attacks on them by their opponents, and so for the most part it is uncertain as to how extensively different they are, and whether any constitute entirely different works. These texts include:
A minority of scholars regard the Gospel of Thomas as part of the tradition from which the canonical gospels eventually emerged; in any case both of these documents are important as showing us what the theoretical Q document might have looked like.
Although there are three texts which take Bartholomew's name, it may be the case that one of the Questions of Bartholomew or the Resurrection of Jesus Christ is in fact the unknown Gospel of Bartholomew.
A number of texts aim to provide a single harmonization of the canonical gospels, that eliminates discordances among them by presenting a unified text derived from them to some degree. The most widely-read of these was the Diatessaron. Of all the extant texts, the majority appear to be variations on the suppressed Diatessaron.
In the modern era, many Gnostic texts have been uncovered, especially from the Nag Hammadi library. Some texts take the form of an expounding of the esoteric cosmology and ethics held by the Gnostics. Often this was in the form of dialogue in which Jesus expounds esoteric knowledge while his disciples raise questions concerning it. There is also a text, known as the Epistula Apostolorum, which is a polemic against Gnostic esoterica, but written in a similar style as the Gnostic texts.
The Sethians were a gnostic group who originally worshipped the biblical Seth as a messianic figure, later treating Jesus as a re-incarnation of Seth. They produced numerous texts expounding their esoteric cosmology, usually in the form of visions:
Several texts concern themselves with the subsequent lives of the apostles, usually with highly supernatural events. Almost half of these are said to have been written by Leucius Charinus (known as the Leucian Acts), a companion of John the apostle. The Acts of Thomas and the Acts of Peter and the Twelve are often considered Gnostic texts. While most of the texts are believed to have been written in the 2nd century, at least two, the Acts of Barnabas and the Acts of Peter and Paul are believed to have been written as late as the 400s.
There are several texts which would be considered part of the apocrypha, which are mentioned in many ancient sources, but for which no known text has survived:
Gospel of Eve (a quotation from this gospel is given by Epiphanius (Haer. xxvi. 2, 3). It is possible that this is the Gospel of Perfection which he alludes to in xxvi. 2. The quotation shows that this gospel was the expression of complete pantheism)
While many of the books listed here were considered heretical (especially those belonging to the gnostic tradition--as this sect was considered heretical by most Christians of the early centuries), others were not considered particularly heretical in content, but in fact were well accepted as significant spiritual works. They are however not considered canonical, as they belong to the category of works of the church fathers or Apostolic Fathers.
The ordinary stand of mainstream Christians in regard to the books deemed apocryphal was succinctly summed up by Robert M. Grant, claiming to speak not as a theologian but as a historian, in the introduction to A Historical Introduction to the New Testament (1963): Aside from the twenty-seven books in the canon, Grant asserted, "No other literature has anything of value to say about Christian origins and the earliest Christian movement." .
J. Quasten, a scholar of early Christian literature (Patrology, 1990) to summarize his view on the Apocryphal literature of early Christianity, quotes M.R. James saying:
People may still be heard to say, 'After all, these Apocryphal Gospels and Acts, as you call them, are just as interesting as the old ones. It was only by accident or caprice that they were not put into the New Testament'. The best answer (...) has always been, and is now, to produce the writings and let them tell their own story. It will very quickly be seen that there is no question of anyone's having excluded them from the New Testament: they have done that for themselves.
However, among historians of early Christianity the books are considered invaluable, especially those which almost made it into the final canon such as Shepherd of Hermas. Bart Ehrman, for example, has said:
The victors in the struggles to establish Christian Orthodoxy not only won their theological battles, they also rewrote the history of the conflict; later readers then naturally assumed that the victorious views had been embraced by the vast majority of Christians from the very beginning ... The practice of Christian forgery has a long and distinguished history ... the debate lasted three hundred years ... even within "orthodox" circles there was considerable debate concerning which books to include -Lost Christianities, Bart Ehrman