Tertullian, Against Praxeas (Circa AD 200-220)
“Ita connexus Patris in Filio et Filii in Paracleto, tres efficit coharentes, alterum ex altere, qui tres unum sunt, non unus, quomodo dictum est, Ego et Pater unum sumus.” (Against Praxeas XXV).
“Thus the connection of the Father in the Son, and of the Son in the Paraclete, produces three coherent persons, one from the other, which three are one, not one [person], as it is said, “I and my Father are One.” (Translation by or for KJV Today)
Thus the connection of the Father in the Son, and of the Son in the Paraclete, produces three coherent Persons, who are yet distinct One from Another. These Three are, one essence, not one Person, as it is said, “I and my Father are One,” [in respect of] unity of substance not singularity of number. (Roberts-Donaldson Translation)
Tertullian alludes to the “Comma” in De Baptismo (translation by S. Thelwall), Chapter VI (concerning the baptismal formula)
Not that in the waters we obtain the Holy Spirit; but in the water, under (the witness of) the angel, we are cleansed, and prepared for the Holy Spirit. In this case also a type has preceded; for thus was John beforehand the Lord's forerunner, “preparing His ways.” Thus, too, does the angel, the witness of baptism, “make the paths straight” for the Holy Spirit, who is about to come upon us, by the washing away of sins, which faith, sealed in (the name of) the Father, and the Son, and the Holy Spirit, obtains. For if “in the mouth of three witnesses every word shall stand:”--while, through the benediction, we have the same (three) as witnesses of our faith whom we have as sureties of our salvation too--how much more does the number of the divine names suffice for the assurance of our hope likewise! Moreover, after the pledging both of the attestation of faith and the promise of salvation under “three witnesses,” there is added, of necessity, mention of the Church; inasmuch as, wherever there are three, (that is, the Father, the Son, and the Holy Spirit,) there is the Church, which is a body of three.
Phoebadius of Agen (France), who died circa AD 392, wrote Contra Arianos around AD 357. It is in Latin.
“Hoc si cui scandalum facit, audiat a nobis Spiritum esse de Deo, quia illi cui est in Filio secunda persona, est et tertia in Spiritu Sancto. Denique Dominus: Petam, inquit, a Patre meo, et alium advocatum dabit vobis. Sic alius a Filio Spiritus; sicut alius a patre Filius. Sic tertia in Spiritu ut in Filio secunda persona, unus tamen omnia quia tres unum sunt.” (Contra Arianos XXVII:4-5)English translation:
“If anyone stumbles at this, let us hear that the Spirit is from God; because that is the second person of the Son, and the third is the Holy Spirit. Finally, the Lord says this, I ask of my Father and he will give you another Advocate. So, the other Spirit comes from the Son; the other Son comes from the Father. So, the Spirit is the third as the Son is the second person. But the one is all, for the three are one.”
“4. If anyone is offended at this, let him also hear us say that the Spirit is from God, since [God] not only has a second person in the Son, but also a third [person] in the Holy Spirit. Our Lord speaks to this: I will ask of my Father and he will give you another Counselor. 5. Just as another – the Son – comes from the Father, so also another – the Spirit – comes from the Son. And just as the Son is the second person [of the Godhead], so also the Spirit is the third. Nevertheless, the sum (omnia) is one God, because the three are one. (Liber Contra Arianos, Kenneth C. Wessel translation)[Note: Heinrich A. W. Meyer believes Phoebadius’s reference may be to Tertullian rather than the apostle John. (Critical and Exegetical Handbook on the General Epistles of James, Peter, John, and Jude. Joh. Ed. Huther, New York, NY: Funk & Wagnalls, 1887, p. 598)]
Gregory of Nazianzus (c. 329 - 390 AD) may allude to the “Comma” in his doxology at the end of Oration 45: The Second Oration on Easter, Paragraph 30:
“Εἰ δὲ καταλύσαιμεν ἀξίως τοῦ πόθου, καὶ δεχθείημεν ταῖς οὐρανίαις σκηναῖς, τάχα σοι καὶ αὐτόθι θύσομεν δεκτὰ ἐπὶ τὸ ἅγιόν σου θυσιαστήριον, ὦ Πάτερ, καὶ Λόγε, καὶ Πνεῦμα τὸ ἅγιον· ὅτι σοὶ πρέπει πᾶσα δόξα, τιμὴ, καὶ κράτος, εἰς τοὺς αἰῶνας τῶν αἰώνων. Ἀμήν.”
“But if we are to be released, in accordance with our desire, and be received into the Heavenly Tabernacle, there too it may be we shall offer You acceptable Sacrifices upon Your Altar, to Father and Word and Holy Ghost; for to You belongs all glory and honour and might, world without end. Amen.” (English translation at New Advent)
In Oration 31, Chapter 19, Gregory of Nazianzus had commented on the unconventional grammar of 1 John 5:6-8 in manuscripts without the “Comma”.
See also “The Greek Text in Acts xx. 28; 1 Timothy iii. 16; and i John v. 7, 8,” The Boston Review, Devoted to Theology and Literature, Volume IV, No. XXI, Boston, MA: John M. Whittemore and Co., 1864, pp. 265-270.