Because they are members of the Body whose Head is Christ, Christians contribute to building up the Church by the constancy of their convictions and their moral lives. The Church increases, grows, and develops through the holiness of her faithful, until "we all attain to the unity of the faith and of the knowledge of the Son of God, to mature manhood, to the measure of the stature of the fullness of Christ."
Source: Catechism of the Catholic Church (Vatican.va)