The gospels they kept in, the ones they left out, the ones they repeated, changed, reinterpreted, twisted and selected among on the basis of credibility (some of the originals were kind of whacko), sense, historical significance, accuracy and adherence to a mainstream belief that may have had little to do with the preaching of the actual Jewish troublemaker they worshipped, made Christianity a straight and narrow path on which the Christian Church could require believers to walk, lest they die.
It's not too hard to keep control of your sources of information when they all have to be written by hand using pens made from dead animal parts.
Pope Innocent III (kind of a misnomer, honestly), banned unauthorized, localized translations of the bible in 1199, as a way to control the message while chopping up and burning those who carried the wrong one.
Translating into a language you could already understand the words of the being that created the world in which you live, whose principles you must live by or risk eternal damnation became heresy punishable by death.
It was a crazy decision, but hardly unique. Even now some Islamic governments and many conservative Muslim scholars consider it to be heresy (on pain of death, seriously) to translate the Quran from Arabic into other languages. If you look at translation as messing with the direct word of the being that created the universe, it probably makes sense.
If you figure the only reason to write a book is to make it possible for others to read what's in it, calling it a sin to distribute or translate a book is an even bigger sin.