Once you believe in God, you belong to other believers. Our relationship with God was not designed to be a secret. It has to be shared to be real. We see in the Bible and in history that spiritual lone rangers don't make it. The way the Bible describes it the church is a body, and the Christians are the limbs and organs. If you cut off one of the pieces, the body suffers, but the piece dies. That is what happens if you cut yourself off from the church. You were created with a special role, and without you fulfilling that role our church cannot be complete.

The Bible makes it clear that a person is to take part in a local fellowship [church], not just attend. In fact, in the Bible, the people who weren't a part of the local fellowship were those who were kicked out for public sins. Attendance is not enough. You also need to participate and love others in the fellowship in order to grow, or you will eventually stop growing and separate yourself from God.
Having a church family identifies you as a believer, pulls you away from selfishness (thinking its about you), and helps to develop spiritual growth. The church also needs you. You were called to share your gifts, talents, experiences, and everything else to complete the church, and also to show love and to share the message of God to unbelievers. If you aren't a part of a church fellowship (big church), small group, and ministry, you will eventually stop growing and separate yourself from God.
Commitment to a church is part of fulfilling God's plan for our lives, so we will never be truly complete and content until we do. The Bible makes it clear that God expects us to do this.

"That’s plain enough, isn’t it? You’re no longer wandering exiles. This kingdom of faith is now your home country. You’re no longer strangers or outsiders. You belong here, with as much right to the name Christian as anyone. God is building a home. He’s using us all-irrespective of how we got here-in what he is building." -Ephesians 2.19 [the Message]
The Bible also gives us a picture of what happens if we aren't committed to a church.

"In this way we are like the various parts of a human body. Each part gets its meaning from the body as a whole, not the other way around. The body we’re talking about is Christ’s body of chosen people. Each of us finds our meaning and function as a part of his body. But as a chopped-off finger or cut-off toe we wouldn’t amount to much, would we? So since we find ourselves fashioned into all these excellently formed and marvelously functioning parts in Christ’s body, let’s just go ahead and be what we were made to be, without enviously or pridefully comparing ourselves with each other, or trying to be something we aren’t." -Romans 12.4-6 [the Message]
Please take this to heart and commit yourselves to each other, as God commands us.

Thinking about my purpose:
Know. I have to belong, not just believe.
Remember. "In Christ we who are many form one body, and each member belongs to all the others." -Romans 12.5 (NIV)
Think about it. Does my level of involvement in my local church show that I love and am committed to God's family?