Irenaeus combatted the gnostic heresy, which taught a multiplicity of gods,
in the late second century.
“But Matthew says that the Magi, coming from the east, exclaimed 'For we have seen His star in the east, and are come to
worship Him;' and that, having been led by the star into the house of Jacob to Emmanuel, they showed, by these gifts which they offered,
who it was that was worshipped; myrrh, because it was He who should die and be buried for the mortal human race; gold, because He was a
King, 'of whose kingdom is no end;' and frankincense, because He was God, who also 'was made known in Judea,' and was 'declared to those who sought
Him not.'” (Irenaeus, Against Heresies, Book 3, Chapter 9.2).
“This is the mystery which he says was made known to him by revelation, that He who suffered under Pontius Pilate, the same is Lord of all, and King, and God, and Judge, receiving power from Him who is the God of all, because He became 'obedient unto death, even the death of the cross.'” (Irenaeus, Against Heresies, Book 3, Chapter 12.9).
"By which is made manifest, that all things which had been foreknown of the Father, our Lord did accomplish in their order, season, and hour, foreknown and fitting, being indeed one and the same, but rich and great. For He fulfills the bountiful and comprehensive will of His Father, inasmuch as He is Himself the Savior of those who are saved, and the Lord of those who are under authority, and the God of all those things which have been formed, the only-begotten of the Father, Christ who was announced, and the Word of God, who became incarnate when the fullness of time had come, at which the Son of God had to become the Son of man." (Irenaeus, Against Heresies, Book 3, Chapter 16.7)
"But that He is Himself in His own right, beyond all men who ever lived, God, and Lord, and King Eternal, and the Incarnate Word, proclaimed by all the prophets, the apostles, and by the Spirit Himself, may be seen by all who have attained to even a small portion of the truth. Now, the Scriptures would not have testified these things of Him, if, like others, He had been a mere man. . . .and that He is the holy Lord, the Wonderful, the Counselor, the Beautiful in appearance, and the Mighty God, coming on the clouds as the Judge of all men;— all these things did the Scriptures prophesy of Him." (Irenaeus, Against Heresies, Book 3, Chapter 19.2).
“'Therefore the Lord himself shall give you a sign; Behold, a virgin shall conceive, and bear a son; and ye shall call His name Emmanuel. Butter and honey shall He eat:. . . .' Carefully, then, has the Holy Ghost pointed out, by what has been said, His birth from a virgin, and His essence, that He is God (for the name Emmanuel indicates this). And He shows that He is a man, when He says, 'Butter and honey shall He eat;' and in that He terms Him a child also, [in saying,] 'before He knows good and evil;' for these are all the tokens of a human infant.” (Irenaeus, Against Heresies, Book 3, Chapter 21.4).
“He says, 'have ye not read that which was spoken by God, saying, I am the God of Abraham, the God of Isaac, and the God of Jacob?' And He added, 'He is not the God of the dead, but of the living; for all live to Him.' By these arguments He unquestionably made it clear, that He who spake to Moses out of the bush, and declared Himself to be the God of the fathers, He is the God of the living. [...] But our Lord is Himself the resurrection, as He does Himself declare, 'I am the resurrection and the life.' [...] Christ Himself, therefore, together with the Father, is the God of the
living, who spake to Moses, and who was also manifested to the fathers.” (Irenaeus, Against Heresies, Book 4, Chapter 5.2)
“...He did Himself declare to those who were accusing His disciples of
not observing the tradition of the elders: 'Why do ye make void the law
of God by reason of your tradition? For God said, Honor thy father and
mother; and, Whosoever curseth father or mother, let him die the death.'
And again, He says to them a second time: 'And ye have made void the word
of God by reason of your tradition;' Christ confessing in the plainest
manner Him to be Father and God, who said in the law, 'Honor thy father
and mother; that it may be well with thee.' For the true God did confess the commandment of the law as the word of God, and called
no one else God besides His own Father.” (Irenaeus, Against Heresies, Book
4, Chapter 9.3).
“Therefore, by remitting sins, He did indeed heal man, while He also manifested Himself who He was. For if no one can forgive sins but God alone, while the Lord remitted them and healed men, it is plain that He was Himself the Word of God made the Son of man, receiving from the Father the power of remission of sins; since He was man, and since He was God, in order that since as man He suffered for us, so as God He might have
compassion on us, and forgive us our debts, in which we were made debtors
to God our Creator.” (Irenaeus, Against Heresies, Book 5, Chapter 17.3).
Hippolytus of Rome
Hippolytus was an 'anti-pope,' the unrecognized bishop of Rome. The heretic
Callistus I holds his place in line by the Roman Catholic Church's reckoning:
"Let us look next at the apostle's word: 'Whose
are the fathers, of whom as concerning the flesh Christ came,
who is over all, God blessed for ever.' This word declares the
mystery of the truth rightly and clearly. He who is over
all is God; for thus He speaks boldly, 'All things are delivered
unto me of my Father.' He who is over all, God blessed, has
been born; and having been made man, He is (yet) God for ever. For to this effect John also has said, 'Which is, and which was,
and which is to come, the Almighty.' And well has he named Christ the Almighty.
For in this he has said only what Christ testifies of Himself."
(Hippolytus, Against the Heresy of One Noetus, Chapter 6.)
Theophilus of Antioch
"But the moon wanes monthly, and in a manner dies, being a
type of man; then it is born again, and is crescent, for a pattern of the
future resurrection. In like manner also the three days which were before
the luminaries, are types of the Trinity, of God, and His Word, and His
wisdom." (Theophilus, To Autolycus, Book 2, Chapter 15).
Clement of Alexandria
This writer, born in mid-second century, headed the catechical school at
"This Word, who alone is both God and man, the cause of all our good, appeared but lately in His own person
to men; from whom learning how to live rightly on earth, we are brought on our way to eternal life." (Clement, Exhortation to the
Greeks, Chapter 1, p. 17 Loeb edition).
"And they rage round the bit of flesh, which they despise as weak,
while they are blind to the inner possessions, not knowing how great a
'treasure' we carry 'in an earthen vessel,' fortified by the power of God the Father and the blood of God the Son and the
dew of the Holy Spirit." (Clement, The Rich Man's Salvation, Section 34, p. 343 Loeb edition).
"For not without divine care could so great a work have been accomplished,
as it has been in so short a time by the Lord, who to outward seeming is
despised, but in very deed is adored; who is the real Purifier, Saviour
and Gracious One, the Divine Word, the truly most manifest God, who is made equal to the Master of the universe, because He was
His Son and 'the Word was in God.'" (Clement, Exhortation to the Greeks, Chapter X, p. 235 Loeb edition).
"What else is necessary? Behold the mysteries of love, and then you will have a vision of the bosom of the Father,
whom the only-begotten God alone declared. God in His very self is love, and for love's sake He became visible to us." (Clement,
The Rich Man's Salvation, Section 37, p. 347 Loeb edition).
Tertullian wrote in the early third century. This Pentecostal author describes
Father, Son and Holy Spirit as being of one substance: "...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..." (Tertullian, Against Praxeas, Chapter II).
This phraseology was later taken up by the Nicene Creed.
“Now although Christ is God, yet, being also man, 'He died according to the Scriptures,' and 'according to the same
Scriptures was buried.'” (Tertullian, A Treatise on the Soul, Chapter 55).
"We who believe that God really lived on earth, and took upon Him the low estate of human form, for the purpose of man’s
salvation, are very far from thinking as those do who refuse to believe
that God cares for anything." (Tertullian, Five Books Against Marcion,
Book 2, Chapter 16)
"By God, however, would that be done which the man Christ was to do, for He was likewise God." (Tertullian, Five Books Against Marcion, Book 4, Chapter 12)
“Moreover, he expressly called Christ God, saying: 'Of whom are the fathers, and of whom as concerning the flesh Christ came, who is over all, God blessed for ever.'” (Tertullian, Against Praxeas, Chapter 15).
“And as for the Father’s names, God Almighty, the Most High, the Lord of hosts, the King of Israel, the 'One that is,' we say (for so much do the Scriptures teach us) that they belonged suitably to the Son also, and that the Son came under these designations, and has always acted in them, and has thus manifested them in Himself to men. 'All things,' says He, 'which the Father hath are mine.' Then why not His names also? When, therefore, you read of Almighty God, and the Most High, and the God of hosts, and the King of Israel the 'One that is,' consider whether the Son also be not indicated by these designations, who in His own right is God Almighty, in that He is the Word of Almighty God, and has received power over all;
is the Most High, in that He is 'exalted at the right hand of God,' as
Peter declares in the Acts; is the Lord of hosts, because all things are
by the Father made subject to Him; is the King of Israel because to Him
has especially been committed the destiny of that nation; and is likewise
'the One that is,' because there are many who are called Sons, but are
not.” (Tertullian, Against Praxeas, Chapter 17).
Tertullian is the author credited with first publication of the word 'Trinity' in Latin: