Heidelberg Catechism - Lord's Day 8

Q & A 24 

Q. How are these articles divided? 

A. Into three parts: 

God the Father and our creation;
God the Son and our deliverance;
and God the Holy Spirit and our sanctification. 

Q & A 25 

Q. Since there is only one divine being,
why do you speak of three:
Father, Son, and Holy Spirit? 

A. Because that is how 

God has revealed himself in his Word:
these three distinct persons
are one, true, eternal God. 

1 Deut. 6:41 Cor. 8:4, 6
2Matt. 3:16-1728:18-19Luke 4:18 (Isa. 61:1); John 14:2615:262 Cor. 13:14Gal. 4:6Tit. 3:5-6