Q. 25 Since there is but one God, 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.
Deut. 6:4; 1Cor. 8:4,6;
Matt. 3:16-17; 28:19-19; Luke 4:18 (Is a. 61:1); John 14:26; 15:26; 2Cor. 13:14;
Gal. 4:6; Tit. 3:5-6