Well there is that which are called mysteries, even things which are already spoken of which God gives us insight into. I never heard of this mystery doctrine, but we know they were stewards of the mysteries of God. Even as Paul speaks of that which was written of the two becoming one (back in Genesis) he is speaking along the lines of Christ and the church, so he hints what is shown plainly (on the one hand) contains more then what meets the eye and speaking in this mystery of Christ in you just the same.
Even in carnal things there are mysteries to me, I mean, a lot of what my own dogs did or do can be a mystery to me, or even what my husband is taken in by, and the scripture speaks of mysteries, I just never heard of a mysetry doctrine. I understand what you mean in that. I recall once someone calling what is "called" (meaning some denomination) "The church of Christ" heretical (in some sense) and I stood there hearing this for the first time (not understanding that the true words for church of Christ is adopted in name to another organization (which is shortsighted somehow) and I just find peoples wording difficult to follow, and so I need to guess half the time, "Are they speaking of a specific church or doctrine they deem heretical or will they just snag you on a word to condemn you" (which seems very common from the earliest of days even till now).
So sometimes you dont know what someone is talking about. I was going to ask what is meant by this doctrine, but then I keep reminding myself not to ask questions in this climate (which is hard for me to remember not to do) but thats a bad habit I am dealing with (Im working on it).