Define “Christian”...

I asked a friend a simple question, so I thought, in the middle of a conversation: “Are they Christian?”
Her answer wasn’t “yes” or “no”, but: “Define Christian…”
A little odd at first it seemed to me, but then I began thinking…
Well, how should I define myself…what did I mean by asking that, and …can one really know for sure about another if they’re Christian…?
For a minute there I sat just thinking, you clearly can’t define a Christian by saying they attend a church, or that their family was Christian…they dress appropriately, act mature, or even say their Christian… one is not defined a Christian based on if they pray, or fast, or even preach…for all these things can be misleading. What if one’s kind, and sweet, and goodness- never curses… or if one knows the Bible very well and even uses it occasionally for his purpose…what if one says he knows so much about a God…but is HE his God?...What if… one lives his life just thinking he’s a Christian…And can one live accordingly abiding Scriptures, and yet not be a Christian?
What is then…what word, what phrase…what sentence can define a Christian?