To be a Christian is to believe in and follow Jesus. It means both trusting in what he’s done and following his example. We are to both believe in him and to become like him.
Those who believe in Christ have been made new by Christ. Having been forgiven of their sins, they are to make every effort to leave the life they lived before Christ by embracing all that God has for them in Christ.
The Church is the Body of Christ. Each person is a member of that body, brought together in Christ to help one another reach maturity in the faith. When each member gives him or herself to the other members of the Church, the Body of Christ grows healthy and strong.