It is not the purpose of this article to prove the early church fathers believed in the Trinity.  I have undertaken that task here.  Instead, this list of citations is intended to demonstrate that the early church fathers were not Unitarians because each clearly taught that the Son pre-existed His human birth.

It has been alleged that these fathers invented the pre-existence of Christ due to the influence of neo-Platonism or Gnosticism.  It will suffice here to say that such a view is becoming less and less convincing to modern Patristic scholars.  The more that is learned about 2nd Temple Judaism and Christianity in its earliest years, the less likely it seems that the old "history of religions" theories can account for primary Christian doctrines.  I commend to the reader Larry Hurtado's One Lord, One God and Lord Jesus Christ as but two examples of modern scholarship that roundly disputes the outdated "pagan influence" arguments.

These Christian thinkers, some of them writing very early in the Christian era, reflect the teaching of the first Christians. While they are not inspired and should be read with discernment, they are vital links to the earliest Christian interpretations of Scripture, and may well - in the case of Ignatius, for example - reflect direct Apostolic teaching.

Justin Martyr: Dialog with Trypho

http://www.ccel.org/ccel/schaff/anf01.viii.iv.lviii.html

“'He is called God, and He is and shall be God.' And when all had agreed on these grounds, I continued: ‘Moreover, I consider it necessary to repeat to you the words which narrate how He who is both Angel and God and Lord, and who appeared as a man to Abraham, and who wrestled in human form with Jacob, was seen by him when he fled from his brother Esau’”

Ignatius: Epistle to the Tarsians

http://ccel.org/fathers2/ANF-01/anf01-28.htm#P2648_435849

How could such a one be a mere man, receiving the beginning of His existence from Mary, and not rather God the Word, and the only-begotten Son? For "in the beginning was the Word, and the Word was with God, and the Word was God."

Ignatius: Epistle to the Magnesians

http://www.ccel.org/ccel/schaff/anf01.v.iii.html

Jesus Christ, who was with the Father before the beginning of time.

Irenaeus: Against Heresies Book I

http://www.ccel.org/ccel/schaff/anf01.ix.ii.ix.html

Having first of all distinguished these three — God, the Beginning, and the Word — he again unites them, that he may exhibit the production of each of them, that is, of the Son and of the Word, and may at the same time show their union with one another, and with the Father. For ‘the beginning’ is in the Father, and of the Father, while ‘the Word’ is in the beginning, and of the beginning. Very properly, then, did he say, ‘In the beginning was the Word,’ for He was in the Son; ‘and the Word was with God,’ for He was the beginning; ‘and the Word was God,’ of course, for that which is begotten of God is God.”

Irenaeus: Against Heresies Book III

http://www.ccel.org/ccel/schaff/anf01.ix.iv.vii.html

"And again when the Son speaks to Moses, He says, ‘I am come down to deliver this people,’ (Exodus 3:8 - the burning bush). For it is He who descended and ascended for the salvation of men." 

Clement of Alexandria: Fragments Part 1

http://www.ccel.org/ccel/schaff/anf02.vi.iv.ix.html

"There was; then, a Word importing an unbeginning eternity; as also the Word itself, that is, the Son of God, who being, by equality of substance, one with the Father, is eternal and uncreate." 

Tertullian: Against Praxeas

http://www.ccel.org/ccel/schaff/anf03.v.ix.xii.html

"[God speaks in the plural ‘Let us make man in our image’] because already there was attached to Him his Son, a second person, his own Word, and a third, the Spirit in the Word....one substance in three coherent persons. He was at once the Father, the Son, and the Spirit."

Hippolytus: Against Noetus

http://www.ccel.org/fathers2/ANF-05/anf05-18.htm#P3415_1067404

Let us believe then, dear brethren, according to the tradition of the apostles, that God the Word came down from heaven,... He now, coming forth into the world, was manifested as God in a body, coming forth too as a perfect man.

Hippolytus: Dogmatical and Historical Fragments

http://ccel.org/fathers2/ANF-05/anf05-18.htm#P3712_1172813

These things then, brethren, are declared by the Scriptures. And the blessed John, in the testimony of his Gospel, gives us an account of this economy (disposition) and acknowledges this Word as God, when he says, "In the beginning was the Word, and the Word was with God, and the Word was God." If, then, the Word was with God, and was also God, what follows? Would one say that he speaks of two Gods? I shall not indeed speak of two Gods, but of one; of two Persons however, and of a third economy (disposition), viz., the grace of the Holy Ghost. For the Father indeed is One, but there are two Persons, because there is also the Son; and then there is the third, the Holy Spirit. The Father decrees, the Word executes, and the Son is manifested, through whom the Father is believed on. The economy of harmony is led back to one God; for God is One. It is the Father who commands, and the Son who obeys, and the Holy Spirit who gives understanding: the Father who is above all, and the Son who is through all, and the Holy Spirit who is in all. And we cannot otherwise think of one God, but by believing in truth in Father and Son and Holy Spirit. For the Jews glorified (or gloried in) the Father, but gave Him not thanks, for they did not recognise the Son. The disciples recognised the Son, but not in the Holy Ghost; wherefore they also denied Him. The Father's Word, therefore, knowing the economy (disposition) and the will of the Father, to wit, that the Father seeks to be worshipped in none other way than this, gave this charge to the disciples after He rose from the dead: "Go ye and teach all nations, baptizing them in the name of the Father, and of the Son, and of the Holy Ghost." And by this He showed, that whosoever omitted any one of these, failed in glorifying God perfectly. For it is through this Trinity that the Father is glorified. For the Father willed, the Son did, the Spirit manifested. The whole Scriptures, then, proclaim this truth.

But some one will say to me, You adduce a thing strange to me, when you call the Son the Word. For John indeed speaks of the Word, but it is by a figure of speech. Nay, it is by no figure of speech. For while thus presenting this Word that was from the beginning, and has now been sent forth, he said below in the Apocalypse, "And I saw heaven opened, and behold a white horse; and He that sat upon him (was) Faithful and True; and in righteousness He doth judge and make war. And His eyes (were) as flame of fire, and on His head were many crowns; and He had a name written that no man knew but He Himself. And He (was) clothed in a vesture dipped in blood: and His name is called the Word of God." See then, brethren, how the vesture sprinkled with blood denoted in symbol the flesh, through which the impassible Word of God came under suffering, as also the prophets testify to me. For thus speaks the blessed Micah: "The house of Jacob provoked the Spirit of the Lord to anger. These are their pursuits. Are not His words good with them, and do they walk rightly? And they have risen up in enmity against His countenance of peace, and they have stripped off His glory." That means His suffering in the flesh. And in like manner also the blessed Paul says, "For what the law could not do, in that it was weak, God, sending His own Son in the likeness of sinful flesh, condemned sin in the flesh, that the righteousness of the law might be shown in us, who walk not after the flesh, but after the Spirit." What Son of His own, then, did God send through the flesh but the Word, whom He addressed as Son because He was to become such (or be begotten) in the future? And He takes the common name for tender affection among men in being called the Son. For neither was the Word, prior to incarnation and when by Himself, yet perfect Son, although He was perfect Word, only-begotten. Nor could the flesh subsist by itself apart from the Word, because it has its subsistence in the Word. Thus, then, one perfect Son of God was manifested.

Theophilus to Autolycus Book II

http://ccel.org/fathers2/ANF-02/anf02-42.htm#P1669_480398

You will say, then, to me: "You said that God ought not to be contained in a place, and how do you now say that He walked in Paradise? "Hear what I say. The God and Father, indeed, of all cannot be contained, and is not found in a place, for there is no place of His rest; but His Word, through whom He made all things, being His power and His wisdom, assuming the person of the Father and Lord of all, went to the garden in the person of God, and conversed with Adam. For the divine writing itself teaches us that Adam said that he had heard the voice. But what else is this voice but the Word of God, who is also His Son? Not as the poets and writers of myths talk of the sons of gods begotten from intercourse [with women], but as truth expounds, the Word, that always exists, residing within the heart of God. For before anything came into being He had Him as a counsellor, being His own mind and thought. But when God wished to make all that He determined on, He begot this Word, uttered, the first-born of all creation, not Himself being emptied of the Word [Reason], but having begotten Reason, and always conversing with His Reason. And hence the holy writings teach us, and all the spirit-bearing [inspired] men, one of whom, John, says, "In the beginning was the Word, and the Word was with God," showing that at first God was alone, and the Word in Him. Then he says, "The Word was God; all things came into existence through Him; and apart from Him not one thing came into existence." The Word, then, being God, and being naturally produced from God, whenever the Father of the universe wills, He sends Him to any place; and He, coming, is both heard and seen, being sent by Him, and is found in a place.

Origen: de Principiis Book II

http://ccel.org/fathers2/ANF-04/anf04-46.htm#P6684_1358950

Seeing, then, that all things which have been created are said to have been made through Christ, and in Christ, as the Apostle Paul most clearly indicates, when he says, "For in Him and by Him were all things created, whether things in heaven or things on earth, visible and invisible, whether they be thrones, or powers, or principalities, or dominions; all things were created by Him, and in Him; " and as in his Gospel John indicates the same thing, saying, "In the beginning was the Word, and the Word was with God, and the Word was God: the same was in the beginning with God: all things were made by Him; and without Him was not anything made.

Origen: de Principiis Preface

http://www.ccel.org/ccel/schaff/anf04.vi.v.i.html

The holy Apostles, in preaching the faith of Christ, treated with the utmost clarity of certain matters which they believed to be of absolute necessity to all believers...The specific points which are clearly handed down through the Apostolic preaching [are] these: First, that there is one God who created and arranged all things...Secondly, that Jesus Christ himself was born of the Father before all creatures...Although He was God, He took flesh, and having been made man, He remained what He was, God

Novation: A Treatise Concerning the Trinity

http://ccel.org/fathers2/ANF-05/anf05-138.htm#P10444_3289233

And let us therefore believe this, since it is most faithful that Jesus Christ the Son of God is our Lord and God; because "in the beginning was the Word, and the Word was with God, and God was the Word. The same was in the beginning with God."

Pseudo-Gregory Thaumaturgus: A Sectional Confession of Faith

http://ccel.org/fathers2/ANF-06/anf06-14.htm

And we anathematize those who constitute different worships, one for the divine and another for the human, and who worship the man born of Mary as though He were another than the God of God. For we know that "in the beginning was the Word, and the Word was with God, and the Word was God."