The three spies a type of the personhood of the Trinity

Thus also did Rahab the harlot, while condemning herself, inasmuch as she was a Gentile, guilty of all sins, nevertheless receive the three spies, who were spying out all the land, and hid them at her home; [which three were] doubtless [a type of] the Father and the Son, together with the Holy Spirit.

Irenaeus of Lyons
Against Heresies, Book IV
Chapter XX

The heretics separate Christ from the Father as two different Gods

These things did the prophets set forth in a prophetical manner; but they did not, as some allege, [proclaim] that He who was seen by the prophets was a different [God], the Father of all being invisible. Yet this is what those [heretics] declare, who are altogether ignorant of the nature of prophecy.

…The prophets, then, indicated beforehand that God should be seen by men; as the Lord also says, “Blessed are the pure in heart, for they shall see God.”

Irenaeus of Lyons
Against Heresies, Book IV
Chapter XX

Christ joins man to God through the incarnation

Now this is His Word, our Lord Jesus Christ, who in the last times was made a man among men, that He might join the end to the beginning, that is, man to God. Wherefore the prophets, receiving the prophetic gift from the same Word, announced His advent according to the flesh, by which the blending and communion of God and man took place according to the good pleasure of the Father, the Word of God foretelling from the beginning that God should be seen by men, and hold converse with them upon earth, should confer with them, and should be present with His own creation, saving it, and becoming capable of being perceived by it, and freeing us from the hands of all that hate us, that is, from every spirit of wickedness; and causing us to serve Him in holiness and righteousness all our days, in order that man, having embraced the Spirit of God, might pass into the glory of the Father.

Irenaeus of Lyons
Against Heresies, Book IV
Chapter XX

God the Trinity, without angels, created the world

It was not angels, therefore, who made us, nor who formed us, neither had angels power to make an image of God, nor any one else, except the Word of the Lord, nor any Power remotely distant from the Father of all things. For God did not stand in need of these [beings], in order to the accomplishing of what He had Himself determined with Himself beforehand should be done, as if He did not possess His own hands. For with Him were always present the Word and Wisdom, the Son and the Spirit, by whom and in whom, freely and spontaneously, He made all things, to whom also He speaks, saying, “Let Us make man after Our image and likeness;” He taking from Himself the substance of the creatures [formed], and the pattern of things made, and the type of all the adornments in the world.

Irenaeus of Lyons
Against Heresies, Book IV
Chapter XIX

Eucharistic heresies of Marcion and the Gnostics

For some, by maintaining that the Father is different from the Creator, do, when they offer to Him what belongs to this creation of ours, set Him forth as being covetous of another’s property, and desirous of what is not His own.

Those, again, who maintain that the things around us originated from apostasy, ignorance, and passion, do, while offering unto Him the fruits of ignorance, passion, and apostasy, sin against their Father, rather subjecting Him to insult than giving Him thanks.

Irenaeus of Lyons
Against Heresies, Book IV
Chapter XVIII

The Father confesses the Son’s name to be his own and incense is offered up to him

He took that created thing, bread, and gave thanks, and said, “This is My body.” And the cup likewise, which is part of that creation to which we belong, He confessed to be His blood, and taught the new oblation of the new covenant; which the Church receiving from the apostles, offers to God throughout all the world, to Him who gives us as the means of subsistence the first-fruits of His own gifts in the New Testament, concerning which Malachi, among the twelve prophets, thus spoke beforehand: “I have no pleasure in you, saith the Lord Omnipotent, and I will not accept sacrifice at your hands. For from the rising of the sun, unto the going down [of the same], My name is glorified among the Gentiles, and in every place incense is offered to My name, and a pure sacrifice; for great is My name among the Gentiles, saith the Lord Omnipotent;”—indicating in the plainest manner, by these words, that the former people [the Jews] shall indeed cease to make offerings to God, but that in every place sacrifice shall be offered to Him, and that a pure one; and His name is glorified among the Gentiles. But what other name is there which is glorified among the Gentiles than that of our Lord, by whom the Father is glorified, and man also? And because it is [the name] of His own Son, who was made man by Him, He calls it His own.

…so also does the Father confess the name of Jesus Christ, which is throughout all the world glorified in the Church, to be His own, both because it is that of His Son, and because He who thus describes it gave Him for the salvation of men. Since, therefore, the name of the Son belongs to the Father, and since in the omnipotent God the Church makes offerings through Jesus Christ, He says well on both these grounds, “And in every place incense is offered to My name, and a pure sacrifice.” Now John, in the Apocalypse, declares that the “incense” is “the prayers of the saints.”

Irenaeus of Lyons
Against Heresies, Book IV
Chapter XVII

 

One God of both Testaments

For the precepts of an absolutely perfect life, since they are the same in each Testament, have pointed out [to us] the same God, who certainly has promulgated particular laws adapted for each; but the more prominent and the greatest [commandments], without which salvation cannot [be attained], He has exhorted [us to observe] the same in both.

Irenaeus of Lyons
Against Heresies, Book IV
Chapter XII

Christ was very man and very God

He, therefore, who was known, was not a different being from Him who declared “No man knoweth the Father,” but one and the same, the Father making all things subject to Him; while He received testimony from all that He was very man, and that He was very God, from the Father, from the Spirit, from angels, from the creation itself, from men, from apostate spirits and demons, from the enemy, and last of all, from death itself. But the Son, administering all things for the Father, works from the beginning even to the end, and without Him no man can attain the knowledge of God. For the Son is the knowledge of the Father; but the knowledge of the Son is in the Father, and has been revealed through the Son;

Irenaeus of Lyons
Against Heresies, Book IV
Chapter VI