Baptism into Christ — Romans 6 and Matthew 28

Rom 6:3-4 interprets baptism as union with Christ's death and resurrection: "we were buried therefore with him by baptism into death, in order that... we too might walk in newness of life." The sign signifies a real participation.

John Calvin — Institutes of the Christian Religion, and Catholic Church — Catechism of the Catholic Church. Augustine — City of God.

Baptism is commanded in the Great Commission of Matt 28:19 and grounded in the union of Rom 6:5-11. It is the visible entry into the body addressed in 1Cor 12:13.