Look at where Gnosticism was prevalent - in the East, closer to the place of Christ's life and ministry and where the more robust Christian communities existed. Look at Paul's First Letter to the Corinthians, look at the Gospel of John. There are clear indications about further initiations. The Christian religion was always multi-tiered; many were called (psychics, baptized in water) but few were chosen (pneumatics, baptized in fire and the Holy Spirit). The secret books were not freely distributed because they were forbidden from being freely distributed. They were only for the inner circle. Then, as the Church spread westwards, people who didn't have an inner circle began to deny that one even existed. Christianity, like many other things, started to fall to the lowest common denominator.

It's a convincing hypothesis, and there is evidence to support it. Ultimately, I think that the foundation of the teaching of Christ, love, was not meant to be withheld or stratified with initiations, but particularly for people who are agnostic or atheist and looking at the spread of Christianity, Gnosticism helps them make sense of it and see it as another mystery-religion. I don't see it as "just another mystery-religion", but when you talk about early Christianity as history rather than as an article of faith, you have to act as a scientist.

"At first we thought it was just another snake-cult, but now their towers are everywhere..."