The New Testament, primarily the Gospels of Mark, John, Luke and Matthew, were written originally in Koine Greek before being translated into Latin, an official 'canon' was created by the Chalcedonian church, though this official 'canon' continued to be changed and evolve especially with the Great Schism which resulted in the Catholic and Orthodox churches being established, a major pioneer was Irenaeus who first started distinguishing between canon and heretical texts, by saying there were four gospels and that everything else was heresy, essentially picking and choosing which he wanted himself and his followers to learn, read and worship from, he is a leading figure as well in the establishment of orthodoxy in the modern churches.
The Gospel of Thomas was excluded because it was not one of these four gospels, there is nothing to discredit it as a legitimate text in the life and sayings of Jesus Christ, except that it was chosen not to be one by the church in the early AD's, a church which was notoriously corrupt and manipulative.
