According to wikipedia:
Collections of related texts such as letters of the Apostle Paul (a major collection of which must have been made already by the early 2nd century)[4] and the Canonical Gospels of Matthew, Mark, Luke, and John (asserted by Irenaeus of Lyon in the late-2nd century as the Four Gospels) gradually were joined to other collections and single works in different combinations to form various Christian canons of Scripture. Over time, some disputed books, such as the Book of Revelation and the Minor Catholic (General) Epistles were introduced into canons in which they were originally absent. Other works earlier held to be Scripture, such as 1 Clement, the Shepherd of Hermas, and the Diatessaron, were excluded from the New Testament. The Old Testament canon is not completely uniform among all major Christian groups including Roman Catholics, Protestants, the Greek Orthodox Church, the Slavic Orthodox Churches, and the Armenian Orthodox Church.However, the twenty-seven-book canon of the New Testament, at least since Late Antiquity, has been almost universally recognized within Christianity
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This corruption is a well known fact,recognized even by the Christians.
The following are the studies doubting the historical reliability of Gospels.
- Craig Evans, “Life-of-Jesus Research and the Eclipse of Mythology,” Theological Studies 54 (1993) p. 5,
- Charles H. Talbert, What Is a Gospel? The Genre of Canonical Gospels pg 42 (Philadelphia: Fortress Press, 1977).
- “The Historical Figure of Jesus,” Sanders, E.P., Penguin Books: London, 1995, p., 3.
- Fire of Mercy, Heart of the Word (Vol. II): Meditations on the Gospel According to St. Matthew – Dr Erasmo Leiva-Merikakis, Ignatius Press, Introduction
- Grant, Robert M., “A Historical Introduction to the New Testament” (Harper and Row, 1963) http://www.religion-online.org/showchapter.asp?title=1116&C=1230
Ehrman, Misquoting Jesus (2005), p. 46 says that Ancient scribes made errors or alterations.
The Biblical scholar Bart D. Ehrman notes that many current verses were not part of the original text of the New Testament. ”These scribal additions are often found in late medieval manuscripts of the New Testament, but not in the manuscripts of the earlier centuries,” he adds. “And because the King James Bible is based on later manuscripts, such verses “became part of the Bible tradition in English-speaking lands
Answering "Christian Apologetics and Research Ministry" 