There is, and always has been, controversy about what the definition of a Christian is and what orthodox Christian beliefs are. For this reason, various creeds (statements of belief) have been developed throughout the history of the church.

The most commonly accepted creed is the Nicene Creed, which is accepted and used by all branches of Christianity. Another common creed is the Apostles' Creed, which is used by the Roman Catholic church and most Protestant churches (see also this explanation of the creed).

Groups which don't accept the beliefs outlined in these creeds - especially the beliefs that there is only one God and that Jesus Christ is both God and human - are not considered to be Christian by mainstream Christians, though some such groups identify themselves as Christians.

For more information, see the Creeds of Christendom website.