Heidelberg Catechism: Lord’s Day 8

Q: 24. How are these articles divided?
A: Into three parts; the first is of God the Father, and our creation; the second of God the Son, and our redemption; the third of God the Holy Spirit, and our sanctification.

Q: 25. Since there is but one only divine essence, why do you speak of Father, Son, and Holy Spirit?
A: Because God has so revealed himself in his word, that these three distinct persons are the one only true and eternal God.