I have heard of this secret teaching but cannot find evidence of it in the two denominations I have belonged to. What happened to it?

Unfortunately a considerable portion of the esoteric side of Christianity was submerged and lost in the flood of ignorance which swept Europe after the fall of the Roman Empire. The mundane, the simple teachings which

satisfied the multitude, and the crude interpretations and explanations which unquestioning minds found adequate, replaced the high spiritual truths originally safeguarded by the enlightened few. Some fragments did survive but not as part of a systematic doctrine. The long and bitter struggle between learning and ignorance, between enlightenment and superstition, raged for centuries and one of the casualties was spiritual knowledge and wisdom. What emerged from the conflict is sufficient original Christianity to satisfy those who are preconditioned to accept unquestioningly, but not nearly enough to meet the need of the enquiring intellect.