Those who see God are in Him

For as those who see the light are within the light, and partake of its brilliancy; even so, those who see God are in God, and receive of His splendour. But [His] splendour vivifies them; those, therefore, who see God, do receive life.

… For the glory of God is a living man; and the life of man consists in beholding God. For if the manifestation of God which is made by means of the creation, affords life to all living in the earth, much more does that revelation of the Father which comes through the Word, give life to those who see God.

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

We should have frequent Communion

so did the Word give to the people that very precept as to the making of oblations, although He stood in no need of them, that they might learn to serve God: thus is it, therefore, also His will that we, too, should offer a gift at the altar, frequently and without intermission.

Irenaeus of Lyons
Against Heresies, Book IV
Chapter XVIII

After the invocation, the bread is no longer common bread

Then, again, how can they say that the flesh, which is nourished with the body of the Lord and with His blood, goes to corruption, and does not partake of life? Let them, therefore, either alter their opinion, or cease from offering the things just mentioned. But our opinion is in accordance with the Eucharist, and the Eucharist in turn establishes our opinion. For we offer to Him His own, announcing consistently the fellowship and union of the flesh and Spirit. For as the bread, which is produced from the earth, when it receives the invocation of God, is no longer common bread, but the Eucharist, consisting of two realities, earthly and heavenly; so also our bodies, when they receive the Eucharist, are no longer corruptible, having the hope of the resurrection to eternity.

Irenaeus of Lyons
Against Heresies, Book IV
Chapter XVIII

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