It is true that the Church, in it's mission for power and control, destroyed many of the other Teachings and alternate churches. Much has been lost here, and this is regrettable.
However, Jesus constantly spoke in symbol and allegory. He did this precisely because deeper teachings were not meant for the masses or for the profane. That which he taught to his disciples privately involved spiritual practices that induced direct spiritual experience, which could then later be discussed with them, but not possibly be understood by the layman or dabbler without similar direct experience. Initiates understand that throwing someone into the deep end that isn't even sure whether to dip their toes in will simply drown the uninitiated. If you think that the most basic and clearest of spiritual principals were perverted and distorted by the Church and others, what do you think would have been done with deeper and more powerful Teachings? If I teach you how to build a fire, and you then become an arsonist, do you think it would be better if I instead gave you the keys to atomic energy?!
There is actually a practical instructional program given by 'outward' and public Christianity. Mostly, this instruction is ethical in nature and seems rather straightforward and simple. But it is not easy. It requires discipline, a tremendous amount of soul searching, and self control. How much have you mastered this instruction (not you personally, but all who read this)? As you begin to master it, layers of symbol and allegory open themselves to you which was not previously seen or understood. At a certain point, you are ready for the deeper teachings and practices, and these things WILL come to you, either in the form of certain documents, a live teacher, or a group. This is when a type of initiation occurs, more advanced teachings commence, and your journey continues on another level.
All things come to those that are ready for them. We are surrounded by Truth, but see only a fraction. The more you open your eyes, the more you see. And that which the young do not need, require, or can digest, will fly over their heads until their next growth spurt.