Christ's Church has been Trinitarian from the beginning . . .
Ignatius of Antioch
Take care, therefore, to be confirmed in the decrees of the Lord and of the Apostles, in order that in everything you do, you may prosper in body and in soul, in faith and in love, in Son and in Father and in Spirit (Letter to the Magnesians 13:1 [A.D. 110]).
Athenagoras
For, as we acknowledge a God, and a Son his Logos, and a Holy Spirit, united in essence, - the Father, the Son, the Spirit because the Son is intelligence, reason, wisdom of the Father, and the Spirit an effluence, as light from fire; so also do we apprehend the existence of other powers, which exercise dominion about matter, and by means of it (A Plea for the Christians, 2:18 [A.D. 177]).
Irenaeus
For the Son, who is the Word of God, arranged these things beforehand from the beginning, the Father being in no want of angels, in order that He might call the creation into being, and form man, for whom also the creation was made; nor, again, standing in need of any instrumentality for the framing of created things, or for the ordering of those things which had reference to man; while, [at the same time,] He has a vast and unspeakable number of servants. For His offspring and His similitude do minister to Him in every respect; that is, the Son and the Holy Spirit, the Word and Wisdom; whom all the angels serve, and to whom they are subject (Against Heresies 4:7:4 [A.D. 189]).
Theophilus of Antioch
The three days which were before the luminaries, are types of the Trinity, of God, and His Word, and His Wisdom (To Autolycus 2:18 [A.D. 181]).
Clement of Alexandria
And the address in the Timœus calls the creator, Father, speaking thus: ‘Ye gods of gods, of whom I am Father; and the Creator of your works.’ So that when he says, ‘Around the king of all, all things are, and because of Him are all things; and he [or that] is the cause of all good things; and around the second are the things second in order; and around the third, the third,’ I understand nothing else than the Holy Trinity to be meant; for the third is the Holy Spirit, and the Son is the second, by whom all things were made according to the will of the Father (Stromata 5:14 [A.D. 202]).
Hippolytus
A man, therefore, even though he will it not, is compelled to acknowledge God the Father Almighty, and Christ Jesus the Son of God, who, being God, became man, to whom also the Father made all things subject, Himself excepted, and the Holy Spirit; and that these, therefore, are three. But if he desires to learn how it is shown still that there is one God, let him know that His power is one. As far as regards the power, therefore, God is one. But as far as regards the economy there is a threefold manifestation, as shall be proved afterwards when we give account of the true doctrine (Against the Heresy of One Noetus 8 [A.D. 200-210]).
Tertullian
…All are of One, by unity (that is) of substance; while the mystery of the dispensation is still guarded, which distributes the Unity into a Trinity, placing in their order the three Persons – the Father, the Son, and the Holy Ghost: three, however, not in condition, but in degree; not in substance, but in form; not in power, but in aspect; yet of one substance, and of one condition, and of one power, inasmuch as He is one God, from whom these degrees and forms and aspects are reckoned, under the name of the Father, and of the Son, and of the Holy Ghost (Against Praxeus 2 [A.D. 213]).
Origen
…the divine benefits [are] bestowed upon us by Father, Son, and Holy Spirit, which Trinity is the fountain of all holiness…. (On First Principles 1:4:2 [A.D. 225]).
Gregory Thaumaturgus
There is a perfect Trinity, in glory and eternity and sovereignty, neither divided nor estranged. Wherefore there is nothing either created or in servitude in the Trinity; nor anything superinduced, as if at some former period it was non-existent, and at some later period it was introduced. And thus neither was the Son ever wanting to the Father, nor the Spirit to the Son; but without variation and without change, the same Trinity abides forever (Declaration of Faith [circa A.D. 250]).
Methodius
Whence also in this place they are not only said to hymn with their praises the divine substance of the divine unity, but also the glory to be adored by all of that one of the sacred Trinity, which now, by the appearance of God in the flesh, hath even lighted upon earth (Oration Concerning Simeon and Anna 2 [A.D. 300]).
Gregory Nazianzen
But [the faithful] worship the Father and the Son and the Holy Spirit, one Godhead; God the Father, God the Son and (do not be angry) God the Holy Spirit, one nature in three personalities, intellectual, perfect, self-existent, numerically separate, but not separate in godhead (Orations 33 [A.D. 374]).
Epiphanius
[The Antiochenes] confess the Father and the Son and the Holy Spirit to be consubstantial, three hypostases, one essence, one divinity. That is the true faith which has been handed down by the fathers… (Against the Heresies of the Panarians 73 [A.D. 374-377]).
Ambrose of Milan
The substance of the Trinity is, so to say, a common Essence in that which is distinct, an incomprehensible, ineffable Substance. We hold the distinction, not the confusion of Father, Son, and Holy Spirit; a distinction without separation; a distinction without plurality; and thus we believe in Father, Son and Holy Spirit as each existing from and to eternity in this divine and wonderful Mystery: not in two Fathers, nor in two Sons, nor in two Spirits (To Gratian, On the Christian Faith 4:8 [A.D. 381]).
Augustine
As regards this question, then, let us believe that the Father, and the Son, and the Holy Spirit is one God, the Creator and Ruler of the whole creature; and that the Father is not the Son, nor the Holy Spirit either the Father or the Son, but a trinity of persons mutually interrelated, and a unity of an equal essence (On the Trinity 9:1 [A.D. 428]).
