I'm sorry, but this is simply not the case.
The Dictionary &
Bertrand Russell's Definition. As you can see from the first link, one of the definitions of agnostic is actually "One who is doubtful or noncommittal about something." I do agree, however, that agnosticism CAN BE a chosen position; it is not necessarily neutral. However, I maintain that an agnostic does
not HAVE TO deny the existence of God, they merely don't think there's enough info to decide. The best example I can think of off-hand would be the jury in a trial before the trial has taken place. They don't know if the defendent is guilty or innocent because they have not heard the evidence. Some jurors will already believe the defendent is guilty, some will already believe s/he is innocent, while others will simply not make a decision because they have not heard the evidence.
I agree that you cannot be an "agnostic Christian," although I think that's entirely based upon the definition of a Christian. The Bertrand Russell article above actually has a comment on this that seems pretty accurate to me. Personally, I think a Christian must believe in the existence of God (
Hebrews 11:6 NIV) and could therefore not be agnostic.
As to the "thing" that all Christian denominations have in common, I personally think you are a little broad. After all Judaism, Christianity, and Islam all believe in the same God, but are clearly not all Christian denominations.
The crux of Christianity is Christ, therefore a Christian denomination must have some belief in Christ, his life, death, and resuccection. To what extent varies largely by the denomination and the individual. I would also add that to actually be a Christian, belief in God's existence is not enough. "You believe that there is one God. Good! Even the demons believe that–and shudder." (James 2:19 NIV) So while belief in God is necessary, I don't think it is a complete definition of a Christian.