The Heidelberg Catechism, This Lord's Day week 8

Q24: How are these articles divided?

A24: 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 Ghost and our sanctification.[1]

1. I Peter 1:2; I John 5:7

Q25: Since there is but one Divine Being,[1] why do you speak of three persons: Father, Son and Holy Ghost?

A25: Because God has so revealed Himself in His Word,[2] that these three distinct Persons are the one, true, eternal God.
1. Deut. 6:4
2. Isa. 61:1; Psa. 110:1; Matt. 3:16-17; 28:19; I John 5:7; II Cor. 13:14