EPT1) Revelation is dated by most scholars both liberal/progressive and evangelical to be around 90-110 CE not 150-200 CE.
2) The books of the new testament were not canonized by a government. Athanasius of Alexandria a 4th century bishop is the first person to have listed the 27 books of the new testament as being authoritative, however before this the authoritative books were largely assumed to be the 27 we have now.
The roman authorities really could not of edited the widely copied and largely distributed books of the new testament. They would need to track them across the grecio-roman world including all the copies that had been made, collect them all, stop people copying them, then re-issue them to all the christian communities in the empire if they wanted to edit them. There is no reputable scholar even liberal or unbelieving scholars who hold that position.
3) The gospels like the gospel of thomas, judas and so on, were not included because
a) they were written in the 3rd century onwards as opposed to the 1st and early 2nd centuries.
b) They were written by communities that were on the fringes of the christian communities and that didn't have mainstream theological ideas.
c) The authorship of Matt, Mark, Luke, John, Acts, and indeed the rest of the new testament were all written by either eye witnesses such as Matthew and John who were 2 of the 12 disciples, or by companions of eye witnesses/apostles such as Luke who was with Paul on his missionary journey's and no doubt also had access to Peter and the Jerusalem leaders as they regularly traveled to Jerusalem and had correspondence.
Mark was also a companion of Paul and Luke but more importantly of Peter who became essentially the most important leader in the early church and was one of the 12 disciples.
These later gnostic gospels had no credibility in terms of dating, authorship or theology to the early Christian communities. They weren't mindlessly accepting any writings that came their way.
I would recommend people look into more credible sources of scholarship than the Da Vinci code book and movie which practically every scholar laughs at and recognises that its a fictional representation of Christian history with some small facts added to give it more credibility.
