The Heidelberg Catechism
Lord's Day 8

Question 24. How are these articles divided?
Answer. Into three parts; the first is of God the Father, and our creation; [a] the second of God the Son, and our redemption; (b) the third of God the Holy Ghost, and our sanctification. ©

Question 25. Since there is but one only [d] divine essence, why speakest thou of Father, Son, and Holy Ghost?
Answer. Because God hath so [e] revealed himself in his word, that these three distinct persons are the one only true and eternal God.

[a]: Gen 1
(b): 1Pet. 1:18,19
©: 1Pet. 1:21,22
[d]: Deut. 6:4
[e]: Gen. 1:26; Isa. 61:1; John 14:16,17; 1John 5:7; John 1:13; Mat. 28:19; 2Cor. 13:14