I believe what I believe is what makes me what I amThere is a long standing belief that any theological statement not explicitly written in the Bible is "man-made religion" or "the invention of man." This belief is closely linked with a suspicion of anything originating from ancient Christianity. It seems that Rich's chorus/statement about the Creed is not only a reaction to this suspicion but a statement about his own understanding of that Creed. His belief (the Creed) is making him. It is forming him and molding him. It is not a once and done salvation prayer or experience, but a living faith that causes him to be molded into the image of God he was created to be.
I did not make it, no it is making me
It is the very truth of God and not the invention of any man
The chorus is also a statement of humility from Rich. He is not relying on his own interpretation of what Christianity is, but rather on an ancient statement of faith. He is proclaiming his acceptance of something that at one time he may have even thought of as an "invention of man," but now very clearly understands to be direct revelation from God. This revelation was not from him nor even from the men who wrote it, but from the Holy Spirit speaking through fallible human beings in order to articulate an orthodoxy and combat heresy.