The Catechism in Questions & Answers

481. What occurs in Holy Baptism with water?

Through Holy Baptism with water a fundamental change in the relationship between a human being and God comes into being. Through the washing away of original sin, the baptised is led out of his original state of remoteness from God and enters the proximity of God. He becomes a Christian.
Through his faith and profession of Christ, the baptised now belongs to the church of Christ.