The Early Church Fathers on
The Trinity

   Groups such as the Jehovah’s Witnesses and Oneness Pentecostals dispute the doctrine of the Trinity. Jehovah’s Witnesses even go so far as to claim that the doctrine of the Trinity was unknown to the Early Fathers. They specifically claim that Tertullian, Origen and Theophilus of Antioch gave evidence against it
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The following quotes show that the doctrine of the Trinity was indeed alive-and-well before the Council of Nicea:
Polycarp (70-155/160).  Bishop of Smyrna.  Disciple of John the Apostle.
"O Lord God almighty... I bless you and glorify you through the eternal and heavenly high priest Jesus Christ, your beloved Son, through whom be glory to you, with Him and the Holy Spirit, both now and forever" (n. 14, ed. Funk; PG 5.1040).
Justin Martyr (100?-165?).  He was a Christian apologist and martyr.
"For, in the name of God, the Father and Lord of the universe, and of our Savior Jesus Christ, and of the Holy Spirit, they then receive the washing with water" (First Apol., LXI).
Ignatius of Antioch (died 98/117).  Bishop of Antioch.  He wrote much in defense of Christianity.
"In Christ Jesus our Lord, by whom and with whom be glory and power to the Father with the Holy Spirit for ever" (n. 7; PG 5.988).
"We have also as a Physician the Lord our God Jesus the Christ the only-begotten Son and Word, before time began, but who afterwards became also man, of Mary the virgin.  For ‘the Word was made flesh.' Being incorporeal, He was in the body; being impassible, He was in a passable body; being immortal, He was in a mortal body; being life, He became subject to corruption, that He might free our souls from death and corruption, and heal them, and might restore them to health, when they were diseased with ungodliness and wicked lusts." (Alexander Roberts and James Donaldson, eds., The ante-Nicene Fathers, Grand Rapids: Eerdmans, 1975 rpt., Vol. 1, p. 52, Ephesians 7.)
Irenaeus (115-190).  As a boy he listened to Polycarp, the disciple of John.  He became Bishop of Lyons.
"The Church, though dispersed throughout the whole world, even to the ends of the earth, has received from the apostles and their disciples this faith: ...one God, the Father Almighty, Maker of heaven, and earth, and the sea, and all things that are in them; and in one Christ Jesus, the Son of God, who became incarnate for our salvation; and in the Holy Spirit, who proclaimed through the prophets the dispensations of God, and the advents, and the birth from a virgin, and the passion, and the resurrection from the dead, and the ascension into heaven in the flesh of the beloved Christ Jesus, our Lord, and His manifestation from heaven in the glory of the Father ‘to gather all things in one,' and to raise up anew all flesh of the whole human race, in order that to Christ Jesus, our Lord, and God, and Savior, and King, according to the will of the invisible Father, ‘every knee should bow, of things in heaven, and things in earth, and things under the earth, and that every tongue should confess; to him, and that He should execute just judgment towards all...'" (Against Heresies X.l)

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 (The 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]).

…all the Scriptures attest the clear existence of, and distinction in (the Persons of) the Trinity, and indeed furnish us with our Rule of faith….  (ibid. 11[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]).

And under this rule must be brought also the understanding of the sacred Scripture, in order that its statements may be judged not according to the worthlessness of the letter, but according to the divinity of the Holy Spirit, by whose inspiration they were caused to be written (ibid. 4:27).

Now this expression which we employ – ‘that there never was a time when He did not exist’ – is to be understood with an allowance.  For these very words ‘when’ or ‘never’ have a meaning that relates to time, whereas the statements made regarding Father, Son, and Holy Spirit are to be understood as transcending all time, all ages, and all eternity.  For it is the Trinity alone which exceeds the comprehension not only of temporal but even of eternal intelligence; while other things which are not included in it are to be measured by times and ages (ibid. 4:28).

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]).

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]).