A. Because that is how God has revealed himself in his Word:2 these three distinct persons are one, true, eternal God.
Biblical References:
1 Deut. 6:4; 1 Cor. 8:4, 6
2 Matt. 3:16-17; 28:18-19; Luke 4:18 (Isa. 61:1); John 14:26; 15:26; 2 Cor. 13:14; Gal. 4:6; Tit. 3:5-6 GOD THE FATHER