The dogma of the Holy Trinity

253 The Trinity is One. We do not confess three Gods, but one God in three persons, the "consubstantial Trinity". (Council of Constantinople II (553): (DS) Denzinger-Schonmetzer 421) The divine persons do not share the one divinity among themselves but each of them is God whole and entire: "The Father is that which the Son is, the Son that which the Father is, the Father and the Son that which the Holy Spirit is, i.e. by nature one God." (Council of Toledo XI (675): DS 530:26) In the words of the Fourth Lateran Council (1215), "Each of the persons is that supreme reality, viz., the divine substance, essence or nature." (Lateran Council IV (1215): DS 804)

254 The divine persons are really distinct from one another. "God is one but not solitary." (Fides Damasi: DS 71) "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." (Council of Toledo XI (675): DS 530:25) 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." (Lateran Council IV (1215): DS 804) The divine Unity is Triune.

255 The divine persons are relative to one another. Because it does not divide the divine unity, the real distinction of the persons from one another resides solely in the relationships which relate them to one another: "In the relational names of the persons the Father is related to the Son, the Son to the Father, and the Holy Spirit to both. While they are called three persons in view of their relations, we believe in one nature or substance." (Council of Toledo XI (675): DS 528) Indeed "everything (in them) is one where there is no opposition of relationship." (Council of Florence (1442): DS 1330) "Because of that unity the Father is wholly in the Son and wholly in the Holy Spirit; the Son is wholly in the Father and wholly in the Holy Spirit; the Holy Spirit is wholly in the Father and wholly in the Son." (Council of Florence (1442): DS 1331)

256 St. Gregory of Nazianzus, also called the Theologian, entrusts this summary of Trinitarian faith to the catechumens of Constantinople: 

Above all guard for me this great deposit of faith for which I live and fight, which I want to take with me as a companion, and which makes me bear all evils and despise all pleasures: I mean the profession of faith in the Father and the Son and the Holy Spirit. I entrust it to you today. By it I am soon going to plunge you into water and raise you up from it. I give it to you as the companion and patron of your whole life. I give you but one divinity and power, existing one in three, and containing the three in a distinct way. Divinity without disparity of substance or nature, without superior degree that raises up or inferior degree that casts down. . . the infinite co-naturality of three infinites. Each person considered in himself is entirely God. . . the three considered together. . . I have not even begun to think of unity when the Trinity bathes me in its splendor. I have not even begun to think of the Trinity when unity grasps me. . .

 

(St. Gregory of Nazianzus, Oratio 40,41: J.P. Migne, ed., Patroligia Greaca (Paris, 1867-1866) 36,417)

From the Fourth Lateran Council:

1. Confession of Faith

We firmly believe and simply confess that there is only one true God, eternal and immeasurable, almighty, unchangeable, incomprehensible and ineffable, Father, Son and holy Spirit, three persons but one absolutely simple essence, substance or nature. The Father is from none, the Son from the Father alone, and the holy Spirit from both equally, eternally without beginning or end; the Father generating, the Son being born, and the holy Spirit proceeding; consubstantial and coequal, co-omnipotent and coeternal; one principle of all things, creator of all things invisible and visible, spiritual and corporeal; who by his almighty power at the beginning of time created from nothing both spiritual and corporeal creatures, that is to say angelic and earthly, and then created human beings composed as it were of both spirit and body in common. The devil and other demons were created by God naturally good, but they became evil by their own doing. Man, however, sinned at the prompting of the devil.

This holy Trinity, which is undivided according to its common essence but distinct according to the properties of its persons, gave the teaching of salvation to the human race through Moses and the holy prophets and his other servants, according to the most appropriate disposition of the times. Finally the only-begotten Son of God, Jesus Christ, who became incarnate by the action of the whole Trinity in common and was conceived from the ever virgin Mary through the cooperation of the holy Spirit, having become true man, composed of a rational soul and human flesh, one person in two natures, showed more clearly the way of life. Although he is immortal and unable to suffer according to his divinity, he was made capable of suffering and dying according to his humanity. Indeed, having suffered and died on the wood of the cross for the salvation of the human race, he descended to the underworld, rose from the dead and ascended into heaven. He descended in the soul, rose in the flesh, and ascended in both. He will come at the end of time to judge the living and the dead, to render to every person according to his works, both to the reprobate and to the elect. All of them will rise with their own bodies, which they now wear, so as to receive according to their deserts, whether these be good or bad; for the latter perpetual punishment with the devil, for the former eternal glory with Christ.

There is indeed one universal church of the faithful, outside of which nobody at all is saved, in which Jesus Christ is both priest and sacrifice. His body and blood are truly contained in the sacrament of the altar under the forms of bread and wine, the bread and wine having been changed in substance, by God's power, into his body and blood, so that in order to achieve this mystery of unity we receive from God what he received from us. Nobody can effect this sacrament except a priest who has been properly ordained according to the church's keys, which Jesus Christ himself gave to the apostles and their successors. But the sacrament of baptism is consecrated in water at the invocation of the undivided Trinity — namely Father, Son and holy Spirit — and brings salvation to both children and adults when it is correctly carried out by anyone in the form laid down by the church. If someone falls into sin after having received baptism, he or she can always be restored through true penitence. For not only virgins and the continent but also married persons find favour with God by right faith and good actions and deserve to attain to eternal blessedness.