There are two distinct uses of the word Christian - in mostly Christian based western societies the adjective "christian" as applied to behaviour is used synonymously with "charitable/kind/understanding etc." in other words "behaving in the same way Jesus reportedly did when on earth".
Of course it means something rather different in some other parts of the world
I have no issue at all with people who's behaviour is "christian" even if they are atheists, hindus, moslems or buddhists. Strangely, most of them preach a very similar morality.
Then there is a Christian as relating to a follower of Jesus Christ, that is someone who accepts the existance of Jesus and his status as the Son of God and attempts to interpret the messages in the biblical account to guide their life. This goes beyond "christian" behaviour of course and spills over into marriage, law, crime and punishment.
Strangely, being a Christian is no guarantee of being christian.
At the tender age of 10, shortly after my own confirmation as a catholic, I came into conflict with that oldest of all christian establishments. You see my mother was catholic and my father an atheist, albeit one with immense humanity and sense of fairness who risked his life more than once to save others. When told by a priest outright that my father would be unable to enter heaven because of his lack of faith, it occurred to me that the God they were talking about was being somewhat churlish to turn aside someone who's actions were by instinct unfailingly "christian" even if his beliefs were not. After all, as a human I would not deprive a starving man of food because he doesnt like me or accept my word, so why would God?
Even worse, I subsequently failed to find any two so called experts who could give me a consistent steer on the subject and I decided they were all making it up, an opinion that I have not changed to this day after confronting the bewildering variety of interpretations of "the word of God". "They can't all be right", say I, "so I would lay odds that none of them are."
I have been a humanist ever since. I believe you dont find morality in god, you find god in morality and that moral behaviour is actually innate in our species and vital for its well being. Where I suspect I differ from most Christians would be in my definition of morality. I cannot for instance understand how a christian or a Christian could condone the death penalty and oppose women's rights.
For me, the fatal credibility flaw with many "religious types" remains their conviction that they, and only they, will enter heaven and that only the "righteous" will share paradise. Moreover the rest, despite the most petty of sins, would be consigned to eternal damnation. That always struck me as a rather extreme punishment.
If I had to spend an eternity with anyone, I would rather spend it with people I actually like. My idea of Hell would be the only atheist surrounded by fundamentalist christians