24. How are these articles divided (see Lord’s Day 7)?
Into three parts: God the Father and our creation; God the Son and our deliverance; God the Holy Spirit and our sanctification.
25. Since there is but one God, why do you speak of three: Father, Son, and Holy Spirit?
Because that is how God has revealed himself in his Word: these three distinct persons are one, true, eternal God.
Dt 6.4; 1Co 8.4,6
Mt 3.16-17; 28.18-19; Lk 4.18 (Isa 61.1); Jn 14.26; 15.26; 2Co 13.14; Gal 4.6; Tit 3.5-6
From the Heidelberg Catechism.