What is a Christian? Or what is Christianity?

I doubt I can define "Christian" in a way that somebody else won't disagree with it, but I think I can define it in a way that's somewhat clear.

A Christian is a follower of Jesus Christ.

Of course that raises the question of what it means to follow Jesus Christ. First, I think it means that you accept, as true, that Jesus is the Christ. That means Jesus is the fulfillment of God's promise to always have a man on the throne of David. David's dynasty came to an end near the beginning of the Babylonian exile. After that, the prophets (e.g. Jeremiah 33) said that God would fulfill his promise by raising up a descendent of David who would restore the throne. His coming would be associated with a gathering of all of God's people who had been scattered through the ages, and a restoration of national sovereignty for Israel, and there would be peace and prosperity to all the nations. Sickness and death will be done away with, and all wars will end. So to be a follower of Jesus Christ means that you accept that Jesus is the fulfillment of those promises, and you place your hope in him for their fulfillment.

It also means that you submit to him as king. So you strive to live according to his moral teachings. You also subscribe to his worldview. And since he believed in YHWH--the god of Abraham, Isaac, and Jacob, so should you.

It also means that you believe the gospel--the central proclamation taught by those Jesus hand picked to be his apostles. An apostle is somebody who is "sent out," i.e. a missionary. To be an apostle of Jesus Christ means that you were personally acquainted with Jesus, and he commissioned you personally. There were originally twelve of them, but Jesus later commissioned Paul.

Paul preserved, in written form, an oral formula that summarizes what is meant by "the gospel" in 1 Corinthians 15, and he said this is the same gospel taught by the original apostles (e.g. Peter, James, and John) in Jerusalem. It includes these facts:

• That Christ died for sins
• That he was raised from the dead
• That he appeared to the apostles

There is a lot of theological content to the claim that Christ died for sins, and there is debate among Christians as to what it means (e.g. different theories of the atonement), but at a minimum it means that Jesus atoned for sins.

This implies that God imposes moral obligations on people, that people disobey those moral obligations, that God judges people for violating his moral requirements, and that we can receive a pardon on the basis of Jesus' atonement and therefore avoid being judged for our sins. In other words, God shows us mercy. Jesus saves sinners from tne judgment of God.

There is some dispute about what is meant by Jesus rising from the dead. You have Jehovah's Witnesses who deny any physical resurrection and claim that Jesus became a spirit. There are others, like Marcus Borg and John Crossan who claim the resurrection is just a metaphor, not a literal event. In spite of these differences, I'm convinced that the resurrection refers to a literal physical event in which Jesus himself physically rose from the dead and left an empty tomb behind him.

There's a lot more to being a Christian than that, but I think these are the essentials of Christianity. They are what all Christians share in common. It's what defines Christianity and makes it what it is and not something else. Christians differ about a lot of things, but if anybody differs on any of these essentials, then they aren't Christians.