Forgive my humble, and possibly uninformed opinion. As a newcomer to Orthodoxy, I would think that True Doctrine can more accurately be found in the teachings of Orthodoxy, and the writings of the Fathers that have been with the True Faith since the earliest of times. As for Gnosticism being focused on as a heresy, there is much merit there. I have recently come to Orthodoxy from a pentecostal denomination, and gnostism runs rampant there, and in most other protestant traditions if one examines them properly. After all, their traditions are built on the self-revelation of only a few, and then expounded upon based on those few misguided revelations. You can't get more "hidden-knowledge" based than to build a denomination on the few passages of scripture that speaks of "tongues." This is an important heresy to stand against when the majority of "christians," other than the Catholic church has left the history, Tradition, and Apostolic truth of the real Church. Again...this is only my 2 cents, but I hope it contributes in some small way.