I think the thing that is defining is what the Bible says, either from Christ or any of the books following the Gospels. I know there is interdenominational infighting but some, if not all of that, is semantics. Nothing any of the traditional denominations do is necessarily counter to the Bible but the infighting comes from the focus of beliefs.
Those people who caused many of the historical Christian massacres and the like (crusades) were abusing their power and were speaking to and leading an illiterate population and even those who were literate in English, German, French and the other European languages could not read Latin, Greek or Hebrew. These people were simply tools who did what they were told. Hell the pope pre-forgave sin and you could buy forgiveness.
It is many times (other than the above mentioned semantics) the truth when it is said that these people were not Christian or more accurately their leaders were everything but Christian and the followers were generally ignorant pawns acting counter to the teachings of the New Testament without even knowing it.