There were honest people long before there were Christians and there are, God be praised, still honest people people where there are no Christians. It could therefore easily be possible that people are Christians because true Christianity corresponds to what they would have been even if Christianity did not exist.
Christians and Christianity
I agree with Big Jim and not only because I am enthralled by the name Big Jim.
I think christian behavior (ie. turning the other cheek, doing unto others, etc.) is a specific evolutionary survival strategy that clearly works to some degree or it would have died out.