Irenaeus of Lyons, Against Heresies 4.9

All things are of one and the same substance — that is, they come from one and the same God — just as the Lord says to the disciples: "Therefore every scribe who has been trained for the kingdom of heaven is like a householder who brings out of his treasure what is new and what is old" (Matt 13:52). He did not teach that one person brings out the old and another the new; it is one and the same householder. The Lord is that householder, who rules the whole household of God paternally: to the servants and those still undisciplined he gives a law worthy of them, but to the children, justified by faith, he gives fitting precepts, and to the sons he opens his inheritance. He calls his disciples scribes and teachers of the kingdom of heaven, and of them he also says elsewhere to the Jews: "I send you prophets and wise men and scribes... and you will kill some of them and persecute them from town to town" (Matt 23:34). Thus the new and old brought out of the treasure speak, without contradiction, of the two testaments: the old, which was the former legislation, and the new, which is the way of life according to the Gospel. Of this David says: "Sing to the Lord a new song" (Ps 96:1), and Isaiah: "Sing to the Lord a new hymn" (Isa 42:10). "From the ends of the earth his name is praised; his deeds are proclaimed in the islands" (Isa 24:16; 42:12). And Jeremiah says: "Behold, the days are coming, declares the Lord, when I will make a new covenant... not like the covenant I made with their fathers at Mount Horeb" (Jer 31:31). Both testaments, however, are produced by one and the same householder — the Word of God, our Lord Jesus Christ — who spoke with Abraham and Moses, who restored liberty to us in newness and multiplied it, which comes from him.

For he says: "Something greater than the temple is here" (Matt 12:6), and "greater than Solomon... greater than Jonah" (Matt 12:41-42), for he gives to men his presence and resurrection from the dead. But he does not change God, nor does he proclaim another Father; he proclaims the same One who always has more for his household, and as their love for God progresses, he gives more and greater gifts — just as the Lord told the disciples: "You will see greater things than these" (John 1:50), and Paul says: "Not that I have already obtained this or am already perfect... For we know in part and we prophesy in part, but when the perfect comes, the partial will pass away" (1 Cor 13:9-10). So when the perfect comes, we will not see another Father, but this One whom we now long to see, for "Blessed are the pure in heart, for they shall see God" (Matt 5:8). We will not wait for another Christ and Son of God, but for this One who was born of the Virgin Mary, who suffered, in whom we believe and whom we love — just as Isaiah says: "They will say in that day: 'Behold, this is our God; we have waited for him, that he might save us. This is the Lord; we have waited for him; let us be glad and rejoice in his salvation'" (Isa 25:9). And Peter says in his epistle: "Though you have not seen him, you love him. Though you do not now see him, you believe in him and rejoice with joy that is inexpressible" (1 Pet 1:8). We will not receive another Holy Spirit, except this One who is with us and cries, "Abba, Father" (Rom 8:15; Gal 4:6). And in these very things we will have increase and progress, so that we may now enjoy the gifts of God not through a mirror and in riddles, but face to face (1 Cor 13:12). Thus also now, perceiving more than Solomon — which is the advent of the Son of God — we have not learned another God besides the Creator and Maker of all things, who was shown to us from the beginning, nor another Christ, Son of God, except the one who was foretold by the prophets.

For with the New Testament known and foretold by the prophets, the One who was to establish it according to the Father's will was also foretold, and he was manifested to men as God willed, so that believers in him may always progress. And through the testaments there is one salvation and one God. The precepts that form a human being are many, and not few are those who lead a human being to God. To an earthly and temporal king, since he is a man, it is permitted to grant greater advancement to those subject to him; but to God, since he is the same and always wills to bestow greater grace on the human race and to honor with more gifts those who please him — is this not progress? To find another Father besides the One announced from the beginning, and then, besides the one thought to have been found a second time, to find a third — and the same progress from a third to a fourth, and from that another and another — so that, always thinking to progress, such a mind will never stand in God. For it is expelled from the One who is, and turning back it seeks but never finds God; it will always swim in the depth of incomprehensibility, unless, converted through repentance, it returns from where it was cast out, confessing and believing in one God, the Father, the Creator, who was announced by the Law and the prophets, who was testified to by Christ — just as he himself says to those who accused his disciples of not keeping the tradition of the elders: "Why do you break the commandment of God for the sake of your tradition? For God commanded, 'Honor your father and your mother,' and, 'Whoever reviles father or mother must surely die.'... So for the sake of your tradition you have made void the word of God" (Matt 15:3-6). Clearly confessing the Father and God, Christ acknowledges the One who said in the Law: "Honor your father and your mother, that it may go well with you" (Deut 5:16). For the Word of God confessed the precept of the Law; the true God did not call another God except his own Father.