Can you define what a Christian is without mentioning the church? If you can, your definition just might not line up with the Bible's.
Think about it. If you're an orphan, you don't adopt parents; they adopt you. If your adoptive parents are named Smith, you now attend the Smith family dinners with the parents and all the children. You share a bedroom at night with the Smith siblings. When the teacher at school calls out attendance and says, "Smith?" you raise your hand like your older brother did before you and your younger sister will do after you. You do this not because you decided to play the role of "Smith," but because someone went to the orphanage and said, "You will be a Smith." On that day, you became the child of someone and the sibling of others.
Only your name's not Smith. It's Christian, named after the one through whom you were adopted, Christ (Eph. 1:5). Now you're part of the whole family of God. "The one who makes men holy and those who are made holy are of the same family" (Heb. 2:11).
And this is no dysfunctional family, with family members estranged from one another. It's a fellowship. When God "called you into fellowship with his Son Jesus Christ our Lord" (1 Corinthians 1:9), he also called you into "fellowship" with the whole family (1 Corinthians 5:2).
And this is no polite and formal fellowship. It's a body, bound together by our individual decisions but also bound together by far more than human decision—the person and work of Christ. You would be as foolish to say, "I'm not a part of the family," as you would be to cut off your own hand or nose. As Paul said to the Corinthians, "The eye cannot say to the hand, ‘I don't need you!' And the head cannot say to the feet, ‘I don't need you!'" (1 Cor. 12:21).
In short, it's impossible to answer the question "What is a Christian?" without ending up in a conversation about the church; at least, in the Bible it is. Not only that, it's hard to stick with just one metaphor for the church because the New Testament uses so many of them: a family and a fellowship, a body and a bride, a people and a temple, a lady and her children. And never does the New Testament conceive of the Christian existing on a prolonged basis outside the fellowship of the church. The church is not really a place. It's a people—God's people in Christ.
When a person becomes a Christian, he doesn't just join a local church because it's a good habit for growing in spiritual maturity. He joins a local church because it's the expression of what Christ has made him—a member of the body of Christ. Being united to Christ means being united to every Christian. But that universal union must be given a living, breathing existence in a local church.
Except for the rarest of circumstances, a true Christian builds his life into the lives of other believers through the concrete fellowship of a local church. He knows he has not yet "arrived." He's still fallen and needs the accountability and instruction of that local body of people called the church. And they need him.
As we gather to worship God and exercise love and good deeds toward one another, we demonstrate in real life, you might say, the fact that God has reconciled us to himself and to one another. We demonstrate to the world that we have been changed, not primarily because we memorize Bible verses, pray before meals, tithe a portion of our income, and listen to Christian radio stations, but because we increasingly show a willingness to put up with, to forgive, and even to love a bunch of fellow sinners.
You and I cannot demonstrate love, joy, peace, patience or kindness sitting all by ourselves on an island. No, we demonstrate it when the people we have committed to loving give us good reasons NOT to love them, but we do anyway.
The church gives a visual presentation of the gospel when we forgive one another as Christ has forgiven us, when we commit to one another as Christ has committed to us, and when we lay down our lives for one another as Christ laid down his life for us.
Together we can display the gospel of Jesus Christ in a way we just can't by ourselves.
"This is no polite and formal fellowship. It's a body, bound together by our individual decisions but also bound together by far more than human decision—the person and work of Christ.”
"Never does the New Testament conceive of the Christian existing on a prolonged basis outside the fellowship of the church. The church is not really a place. It's a people—God's people in Christ.”
"You and I cannot demonstrate love, joy, peace, patience or kindness sitting all by ourselves on an island. No, we demonstrate it when the people we have committed to loving give us good reasons NOT to love them, but we do anyway.”