CCC 254 The divine persons are really distinct from one another. “God is one but not solitary.”1 “Father”, “Son”, “Holy Spirit” are not simply names designating modalities of the divine being, for they are really distinct from one another: “He is not the Father who is the Son, nor is the Son he who is the Father, nor is the Holy Spirit he who is the Father or the Son.”2 They are distinct from one another in their relations of origin: “It is the Father who generates, the Son who is begotten, and the Holy Spirit who proceeds.”3 The divine Unity is Triune.
1 Fides Damasi: DS 71.
2 Council of Toledo XI (675): DS 530:25.
3 Lateran Council IV (1215): DS 804.