What the Church Fathers have written on the Trinity:Justin Martyr"We will prove that we worship him reasonably; for we have learned that he is the Son of the true God himself, that he holds a second place, and the Spirit of prophecy a third. For this they accuse us of madness, saying that we attribute to a crucified man a place second to the unchangeable and eternal God, the Creator of all things; but they are ignorant of the mystery which lies therein." (First Apology 13:5–6 [A.D. 151]).
Theophilus of Antioch"It is the attribute of God, of the most high and almighty and of the living God, not only to be everywhere, but also to see and hear all; for he can in no way be contained in a place.... The three days before the luminaries were created are types of the Trinity: God, his Word, and his Wisdom." (To Autolycus 2:15 [A.D. 181]).
Irenaeus"For the Church, although dispersed throughout the whole world even to the ends of the earth, has received from the apostles and from their disciples the faith in one God, the Father Almighty ...and in one Jesus Christ, the Son of God, who became flesh for our salvation; and in the Holy Spirit." (Against Heresies 1:10:1 [A.D. 189]).
Tertullian"We do indeed believe that there is only one God, but we believe that under this dispensation, or, as we say, oikonomia, there is also a Son of this one only God, his Word, who proceeded from him and through whom all things were made and without whom nothing was made…. We believe he was sent down by the Father, in accord with his own promise, the Holy Spirit, the Paraclete, the sanctifier of the faith of those who believe in the Father and the Son, and in the Holy Spirit…. This rule of faith has been present since the beginning of the gospel, before even the earlier heretics." (Against Praxeas 2 [A.D. 216]).
"And at the same time the mystery of the oikonomia is safeguarded, for the unity is distributed in a Trinity. Placed in order, the three are the Father, Son, and Spirit. They are three, however, not in condition, but in degree; not in being, but in form; not in power, but in kind; of one being, however, and one condition and one power, because he is one God of whom degrees and forms and kinds are taken into account in the name of the Father, and of the Son, and of the Holy Spirit." (ibid).
"Keep always in mind the rule of faith which I profess and by which I bear witness that the Father and the Son and the Spirit are inseparable from each other, and then you will understand what is meant by it. Observe now that I say the Father is other [distinct], the Son is other, and the Spirit is other. This statement is wrongly understood by every uneducated or perversely disposed individual, as if it meant diversity and implied by that diversity a separation of Father, Son, and Holy Spirit." (ibid., 9).
"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' [John 10:30], in respect of unity of being not singularity of number." (ibid., 25).
Origen"For we do not hold that which the heretics imagine: that some part of the being of God was converted into the Son, or that the Son was procreated by the Father from non-existent substances, that is, from a being outside himself, so that there was a time when he [the Son] did not exist." (The Fundamental Doctrines 4:4:1 [A.D. 225]).
"No, rejecting every suggestion of corporeality, we hold that the Word and the Wisdom was begotten out of the invisible and incorporeal God, without anything corporal being acted upon…the expression which we employ, however that there was never a time when he did not exist is to be taken with a certain allowance. For these very words 'when' and 'never' are terms of temporal significance, while whatever is said of the Father, the Son, and the Holy Spirit, is to be understood as transcending all time, all ages." (ibid.).
"For it is the Trinity alone which exceeds every sense in which not only temporal but even eternal may be understood. It is all other things, indeed, which are outside the Trinity, which are to be measured by time and ages." (ibid.).
Hippolytus"The Word alone of this God is from God himself, wherefore also the Word is God, being the being of God. Now the world was made from nothing, wherefore it is not God." (Refutation of All Heresies 10:29 [A.D. 228]).
Novatian"For Scripture as much announces Christ as also God, as it announces God himself as man. It has as much described Jesus Christ to be man, as moreover it has also described Christ the Lord to be God. Because it does not set forth him to be the Son of God only, but also the son of man; nor does it only say, the son of man, but it has also been accustomed to speak of him as the Son of God. So that being of both, he is both, lest if he should be one only, he could not be the other. For as nature itself has prescribed that he must be believed to be a man who is of man, so the same nature prescribes also that he must be believed to be God who is of God…. Let them, therefore, who read that Jesus Christ the son of man is man, read also that this same Jesus is called also God and the Son of God." (Treatise on the Trinity 11 [A.D. 235]).
Pope Dionysius"Next, then, I may properly turn to those who divide and cut apart and destroy the most sacred proclamation of the Church of God, making of it [the Trinity], as it were, three powers, distinct substances, and three godheads.... [Some heretics] proclaim that there are in some way three gods, when they divide the sacred unity into three substances foreign to each other and completely separate." (Letter to Dionysius of Alexandria 1 [A.D. 262]).
"Therefore, the divine Trinity must be gathered up and brought together in one, a summit, as it were, I mean the omnipotent God of the universe.... It is blasphemy, then, and not a common one but the worst, to say that the Son is in any way a handiwork [creature].... But if the Son came into being [was created], there was a time when these attributes did not exist; and, consequently, there was a time when God was without them, which is utterly absurd." (ibid., 1–2).
"Neither, then, may we divide into three godheads the wonderful and divine unity.... Rather, we must believe in God, the Father Almighty; and in Christ Jesus, his Son; and in the Holy Spirit; and that the Word is united to the God of the universe. 'For,' he says, 'The Father and I are one,' and 'I am in the Father, and the Father in me'." (ibid., 3).
Gregory the Wonderworker"There is one God.... 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 super-induced, 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 ever." (Declaration of Faith [A.D. 265]).
Saint Thomas Aquinas, Patron of Students: Pray for us. 
