[pinkpanther]

Christianity is supposed to be monotheistic but you associate partners to the one true God. So, while you say God is one but in practice, you associate partners with God when you take a man as God.

Declaration of Faith...   a declaration that is meant to free you from all Shirk (polytheism). The purpose of the Shahada is to affirm the Oneness of God and to declare that He has no partners or associates. 

By adding another name, you are, in effect, contradicting the very statement you just made by associating another entity with God.