Besides ordaining ministers, Rabbis, and other clergy, our Seminary also runs an online Mystery School with many fun lessons. One lesson is based on the life-changing book, Inner Christianity: A Guide to the Esoteric Tradition by Richard Smoley.
A current member just answered the Chapter 4 question, “What is the central mystery of Christianity?” as follows:
For one to enter into the Kingdom of God, one must step past the constraints of the “I” and understand that the “I” is ultimately the same in all of us. “For as in Adam all die, even so in Christ shall all be made alive” (1 Cor. 15:22) “God has only one Son. If all His creations are His Sons, every one must be an integral part of the whole Sonship, so the Sonship in its oneness transcends the sum of its parts.” …that there is an underlying unity of Being of which we are all part.