Here are two very different definitions of what one might use to describe what a Christian should believe. I'm going to leave out behavior, because that is subjective and subject to judgement by people who still have a "plank in their eye".
1) From their Faith, a Christian believes in the creed (which is a version of the Nicene Creed) as professed by their denonination/congregation.
2) They have read the three Synoptic Gospels, and strive to live their lives per the teachings of Jesus.
Those two definitions are very different, but not mutually exclusive.
I follow one of those two and not the other... anyone care to guess which one?