The New Testament of the Bible places great importance on assembly and sharing the Christian message. Church is important to a Christian so she can not only develop a relationship with God, but also with other Christians.
The act of assembly is a common theme in the New Testament. The apostles travel from town to town gaining converts and creating small groups, or churches. The converts then use their new-found knowledge to preach to others and create more churches.
Attending a church not only gives Christians a chance to share beliefs, but it also provides a measure of accountability. Interacting with a church official, such as a pastor or priest, on a regular basis keeps the teachings of Christianity in the forefront. A Christian who is accountable only to himself is more apt to slip up and commit sin.
Other Christians point to the fact that the Bible refers to the church as Jesus' bride, to whom he must be faithful. If Jesus must be faithful to the church, how can a Christian do anything different?
The church has played an important role keeping the story of Jesus alive when political powers would have preferred it to be erased. By preserving the scriptures and providing a place for assembly and prayer, the church still plays an important role in the lives of Christians.