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.
Sometimes theologians refer to a distinction between the universal church (all Christians everywhere throughout history) and the local church (those people who meet down the street from you to hear the Word preached and to practice baptism and the Lord's Supper). Other than a few references to the universal church (such as Matthew 16:18 and the bulk of Ephesians 1), most references to the church in the New Testament are to local churches, as when Paul writes, "To the church of God in Corinth" or "To the churches in Galatia."
Now what follows is a little intense, but it's important. The relationship between our membership in the universal church and our membership in the local church is a lot like the relationship between the righteousness God gives us through faith and the actual practice of righteousness in our daily lives. When we become Christians by faith, God declares us righteous. Yet we are still called to actively be righteous. A person who happily goes on living in unrighteousness calls into question whether he ever possessed Christ's righteousness in the first place (see Romans 6:1-18; Romans 8:5-14; James 2:14-15). So, too, it is with those who refuse to commit themselves to a local church. Committing to a local body is the natural outcome - it confirms what Christ has done. If you have no interest in actually committing yourself to an actual group of gospel-believing, Bible-teaching Christians, you might question whether you belong to the body of Christ at all.
Taken from "The Church: A Family, a Fellowship, and the Body of Jesus Christ" (used by permission).