Why doesn't the Coptic Orthodox Church accept the Gospel of Thomas?
The apocryphal Gospel of Thomas was found with the Nag Hammadi excavations in 1947-49. It is written in Coptic and belongs to the Gnostic tradition, dating to around AD200 (long after the martyrdom of St. Thomas). The content of this "gospel" is a collection of sayings by Christ, but little in the way of any actions.
Gnosticism was a philosophical orientation that was strong in the first century AD. It competed with Judaism and Christianity, but eventually failed in the struggle. "Gnosis" simply means "Knowledge." The classical Gnostic view is that salvation derives from an insight into secret knowledge and the capacity to discover truth by means of our intellect alone. The Gnostic view also understands everything as an outcome of a struggle between two powers, light and good versus dark and evil. This dualism is what characterizes Gnosticism.
There were a number of Gnostic writers and writers of other cults and sects in the days of early Christianity that used and reinterpreted Christian theology to suit their own philosophical points of view. The gospel of Thomas is an example of this. St Paul warns against such views and instructs Christians to stay away from them.