I saw another post ask this question, and the usual responses followed: justified in the blood of Jesus, someone who holds the right beliefs (resurrection, correct atonement theory, correct belief of the nature of Jesus).
That's not how I would've answered.
When it's used in the New Testament, it means those who followed Jesus, or those who were disciples of Christ. It's reasonable to infer, therefore, that Christian basically means "Christ-like." "Christ" means the Messiah, or Anointed One. So to be Christian is literally to be "Anointed One-like."
I'm trying to think of an exemption to this, but usually when we say that someone is like another person, it's generally to describe characteristics or behavior. If I am like my friend or family member, it's because I would do what the friend/family member would do, or vice versa. So if we are to be Christ-like, then we would do what Christ did. Otherwise, we reduce "Christ-like" to believe as Jesus believed.
The Gospel of John focuses a lot of Jesus as the divine son of God, who Jesus was and his relation to the world and God, and to believe in the son is to receive eternal life/salvation. Inevitably, that gets used a lot in defining Christian.
Here's my problem: John is one of four books that details the life of Jesus. It's in the minority, and yet it gets used as a majority (I'm referring the books that deal with Jesus when he walked on this planet only right now). The other three books show that Jesus places a huge emphasis on action. The harsh words he had were for the religious elites -- those who clung to the correct beliefs at the expense of all else, including their fellow humans.
Let's take the Samaritan. By all accounts, the Samaritan was a heretic, a sinner, and definitely on the "outs" with God because he had the wrong beliefs. And yet that person was the most "Anointed-One like," because he behaved as an Anointed One should. When the three people would stand before God, who would receive the praise?
When we look at our neighbors today, and compare the behavior of some who call themselves Christian to others who call themselves agnostic ... I can't help but think the agnostics are going to receive the praise, because they truly behaved as though Anointed with the Spirit of God.
No, this is not a matter of "earning" salvation. Repentance and turning from sin can come in a variety of ways. If we work with someone who is an absolute jerk, and the understandable response would be to belittle this person, we are repenting through responding with calmness. We are repenting if we decide to see past the jerk-facade, and into the factors that influenced him as a child, or even still influence him. We repent by looking past the surface. We turn from sin when we refuse to hold onto our anger, or refuse to let "the jerk" influence our response, or refuse to let "the jerk" discolor our ability to love him or her.
So who is a Christian? Someone who is Christ-like. Someone who behaves as Jesus did, who sees as Jesus did.