In my time as a member I have heard many talk of deep mysteries of the gospel. How many angels can you fit on a pin head? Can God scratch his right elbow with his right hand? Does God know how many planets are in existence throughout all eternity? Who was the first God? And the list is endless itself.
Are these "deep doctrine"? Or just fantasies to intellectualise the gospel into a concept for the learned? To spend time giving silly answers to silly questions, while convincing ones self of ones superior intelligence for being in such a superior discussion?
To me, doctrine worth discussing, and that has more chance of bringing out truth, must be doctrine that has some good intent in its asking. Abraham stood before Christ and posed the question, "wilt thou also destroy the righteous with the wicked" (Gen 18:23)? Was this an intellectual or heart felt question? In verse 25 Abraham asks Christ, "Shall not the judge of all the earth do right?" Plainly Abraham had concern in his questions. He wanted to be assured that God is good. Christ assured Abraham that he was. This is the sort of stuff that deep doctrine is made of. Deep spiritual experiences that give us knowledge about God and ourselves.
The deepest doctrine that I feel that exists within the church is that which is so often discussed, but rarely understood: Faith, repentence, baptism and the gift of the Holy Ghost. I have met only a few (and I mean a few) who actually perceive the full significance of this message.
As you get into the habit of discussing deep, heart felt, doctrine you will begin to think in wiser ways. And the Holy Ghost will be with you more, to give you greater understanding.