Summa summae S. Thomae sive compendium theologiae

I say: There is one God, numerically and individually, in three Persons — that is, one and the same individual divine essence in three really distinct Persons: the Father unbegotten, the Son begotten, the Holy Spirit proceeding.

Proof from the Old Testament. 1° Genesis I, 26: God said: Let us make man in our image and likeness. — Behold plurality. And immediately after, v. 27: He created man in his own image. — Behold unity. 2° Genesis XVIII, 1–3: The Lord appeared to him [Abraham] in the valley of Mambre… And when he raised his eyes, there appeared to him three men standing near him; and when he saw them… he bowed to the ground. And said: Lord, if I have found favour in your eyes. Weighing these words, Augustine, Against Maximinus, bk. II, c. 26, n. 5, says: "He sees three, yet calls them not 'lords' but 'Lord,' because the Trinity indeed consists of three Persons, but there is one Lord God."

Proof from the New Testament. 1° Matthew XXVIII, 19: Baptizing them in the name of the Father, and of the Son, and of the Holy Spirit. In these words the unity of essence is indicated by the singular in the name — not in the names — and the Trinity of Persons by the enumeration: of the Father, and of the Son, and of the Holy Spirit. 2° 1 John V, 7: There are three who give testimony in heaven: the Father, the Word, and the Holy Spirit; and these three are one.

Against this most forceful testimony the Socinians attack two things: the authenticity of the text and its meaning. Regarding the authenticity of the text, they say that this seventh verse of chapter V of the First Epistle of John is spurious: (1) because it is not found in the more ancient and well-attested manuscripts; (2) because it was not cited by Athanasius, Hilary, Chrysostom, Ambrose, Gregory Nazianzen, and others against the Arians and Sabellians, even though it would have been most apt for refuting them. To the first objection, we admit that this verse, whether in full or in part, is not found in certain ancient manuscript copies; but it is found in far more of the most ancient and best-attested manuscripts.

That it is absent from a few copies could have occurred: (1) through the treachery of the Arians, who erased this text as too contrary to their error, as Jerome attests in his Prologue to the Catholic Epistles — if indeed that Prologue is Jerome's, a point currently debated by critics. But whatever the case, St. Ambrose frequently reproaches the Arians for this sort of fraud: On the Faith, bk. II, c. 15; bk. III, c. 16; On the Holy Spirit, bk. IV, c. 40. (2) The omission could also have arisen from a copyist's error: having arrived at the words of verse 7, There are three who give testimony, his eye skipped to verse 8, where the same words are repeated. This slip is common among copyists when the same words are repeated with scarcely any interval.

Moreover, our Vulgate edition was made according to the copies which were in everyday use among the faithful. These are far more difficult to corrupt, owing to their daily use and the certain risk of the fraud being detected; they therefore deserve far greater credibility than those that lie hidden in the corners of libraries, where through the negligence or malice of copyists they lie open to tampering and falsification.

To the second objection — that several Fathers did not cite this verse against heretics — I reply that several others did cite it. Tertullian cited it, Against Praxeas, c. 31; St. Cyprian, On the Unity of the Church; Jerome, in his Prologue to the Catholic Epistles; Cassiodorus, Fulgentius, and several others. This positive proof certainly outweighs the negative argument of the Socinians. Hence, that we do not read this verse having been cited in the writings of certain ancients is probably due to the fact that they happened upon a defective codex, or that not all their writings have come down to us, or that they were corrupted by enemies of the faith.¹

Regarding the meaning of this verse, the Socinians say that the words and these three are one do not signify unity of nature or substance, but unity of testimony — because in verse 8 following, of the spirit, the blood, and the water it is likewise said: and these three are one, which is not understood as unity of nature but of testimony, insofar as they concur in the same testimony about Christ, namely that he is the truth, just as the Father, the Son, and the Holy Spirit do. Reply 1°: These words of verse 8, and these three are one, are absent from many Latin codices. The Louvain correctors count fifteen in which they are not read. Innocent III, in the decretal found in Decretals III, tit. 41, c. In quadam, says that "in most codices" — probably Latin ones — "they are not found." The Greek codices do have them, yet not as cited, but thus: and these three are unto one [in unum], that is, they concur in one testimony. It is therefore rightly presumed that, through Arian fraud or copyist negligence, the word in dropped out of the Latin codices that contain these words. But even if our Vulgate does not read it that way, it must still be so explained as to correspond to the Greek original, so that the sense is: the spirit, the water, and the blood concur in attesting the same thing, namely that Christ is true man. And although in both verses there is a certain parallel in rendering testimony, it is not necessary that there be a parallel in the signification of the words and these three are one.

Reply 2°: Granted that these words express a parallel, that parallel is indeed found; for these three — Spirit, Water, and Blood — are in some manner substantially one. Let the sense then be: The Spirit, Water, and Blood gave testimony about Christ at the time of his passion: the Spirit, when he cried out and expired; the Blood and Water flowing from his side. And these three are substantially one, that is, they belong to one and the same human substance in Christ — just as the Father, the Son, and the Holy Spirit belong to one and the same divine substance — with this difference, however, that the divine substance is supremely simple and one, while the human substance is composite. 3° John X, 30: I and the Father are one. I and the Father are — behold the distinction of Persons; one — behold consubstantiality. Hence Augustine, Tract. XXXVI on John ch. VIII, says: "What he said, one, frees you from Arius; what he said, are, frees you from Sabellius." This passage was urged most forcefully by the Fathers against the heretics.

4° What could be plainer than John I, 1: In the beginning was the Word, and the Word was with God, and the Word was God?

Proof of the conclusion from Tradition. 1° From the form of Baptism which has been constantly in use from the very beginning of the Church; this form contains a profession of three Persons in one numerically identical substance, as we have considered above.

2° From the protest of the whole Church against the heresies that arose concerning this mystery — those of Sabellius, Arius, Macedonius, and others. From this it is clear that this is and has been the ancient and constant teaching of the faithful.

3° From the holy Fathers who in the first three centuries before the Council of Nicaea expressed this faith in their writings.

First century. Hermas, disciple of Paul, in the work called The Shepherd, bk. III, Similitudes 5 and 9. St. Clement, fellow worker of Paul and successor of Peter in the Roman See, Epistle to the Corinthians, n. 16 and n. 161 [sic — verify]. St. Dionysius the Areopagite, another disciple of St. Paul, On the Divine Names, c. 1. St. Ignatius Martyr, hearer of John the Evangelist, Epistles to the Ephesians and to the Magnesians.

Second century. St. Polycarp, disciple of both St. John the Evangelist and St. Ignatius, bishop of Smyrna, and with him the entire Church of Smyrna, in the letter written about St. Polycarp's martyrdom to the Churches of Pontus, in Eusebius, Ecclesiastical History, bk. IV, c. 15. St. Justin Martyr, Apology II. Athenagoras, the Christian philosopher, Apology for Christians to the Emperors. St. Irenaeus, disciple of St. Polycarp, bishop of Lyons and martyr, Against Heresies, bk. I, c. 2.

Third century. Clement of Alexandria, Paedagogus, bk. I, c. 6. Tertullian, Against Praxeas, c. 2, and On Modesty, c. 21. St. Cyprian, On the Unity of the Church, c. 4.

Finally, in the fourth century: at the Council of Nicaea in the year 325, against Arius, and at the Council of Constantinople in the year 381, against Macedonius, the Catholic faith concerning the Trinity and the consubstantiality of the Persons in one and undivided God was upheld — a faith which all the faithful have since unanimously professed in the Creed sung by the Church.