Heidelberg Catechism – Lord’s Day 8
Q. How are these articles [the clauses of the Apostles’ Creed] divided?
A. Into three parts: The first concerns God the Father and our creation; the second, God the Son and our redemption; and the third, God the Holy Spirit and our sanctification.
Q. Since there is only one Divine Being, why do you speak of three, Father, Son, and Holy Spirit?
A. Because God has thus revealed himself in his Word, that these three distinct persons are the one, true, eternal God.