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.