Irenaeus of Lyons, Against Heresies 4.12

The elders' own tradition, which they pretended to derive from the Law, stood in direct opposition to the Law that Moses delivered. That is why Isaiah cries out, 'Your wine merchants mix wine with water' (Isaiah 29:13), exposing how they diluted the austere command of God with a watery tradition — in effect, substituting an adulterous law that contradicted the true Law. The Lord Himself made the same charge: 'Why do you transgress the command of God for the sake of your tradition?' (Matthew 15:3). They did not merely frustrate God's Law by this prevarication, mixing wine with water; they also erected a counter-law of their own. In it, some commandments still recognized as Pharisaic are removed, others are added, and still others are twisted to suit the whims of individual teachers. They clung to these traditions precisely to avoid submitting to the Law of God, which was designed to prepare them for Christ's coming. They even accused the Lord of healing on the Sabbath, though the Law never forbade such mercy. After all, they themselves circumcised a man on the Sabbath without self-condemnation, yet they violated God's commandment through their tradition and Pharisaic code, lacking the very heart of the Law: love toward God.

Because love of God is the first and greatest commandment, and love of neighbor the second, Jesus taught that the whole Law and the Prophets hang on these two (Matthew 22:37–40). He did not introduce a new, higher commandment; He renewed this same one for His disciples, ordering them to love God with all their heart and their neighbor as themselves. Had He come from a different Father, He would never have appealed to the Law's first and greatest commandment, but would have brought a superior one from that perfect Father, rather than using what the God of the Law had given. Paul agrees: 'Love is the fulfillment of the Law' (Romans 13:10). When everything else is stripped away, faith, hope, and love remain, and the greatest of these is love (1 Corinthians 13:13). Knowledge without love toward God profits nothing; neither the grasp of mysteries, nor faith, nor prophecy avails — all are empty and futile without love. Love perfects the perfect person, and the one who loves is perfect toward God, both in this age and in the age to come. We never cease loving God; the more we behold Him, the more we love Him.

Therefore, since the Law and the Gospel alike set the first and greatest commandment as loving the Lord God with all your heart, and the second as loving your neighbor as yourself, the same God is revealed as the Author of both. The commandments of perfect life are identical in the two Testaments, and they point to one and the same God. He gave particular commandments suited to each era, but the supreme and essential ones — without which no one can be saved — He kept the same in both. The Lord shames those who claim the Law comes from another God when He tells His disciples, 'The scribes and the Pharisees sit on Moses' seat; so practice and observe whatever they tell you, but not what they do, for they preach but do not practice. They bind heavy burdens and lay them on men's shoulders, but they themselves are not willing to move them with so much as a finger' (Matthew 23:2–4). He was not condemning the Law given through Moses — which He still urged people to keep while Jerusalem stood — but indicting those who recited the Law's words while lacking love, thereby becoming unjust toward both God and neighbor. As Isaiah says, 'This people honors me with their lips, but their heart is far from me; in vain do they worship me, teaching as doctrines the commandments of men' (Isaiah 29:13). They were not following the Law of Moses, but their own invented traditions; by defending these, they nullified God's Law and refused submission to His Word. Paul says of them, 'For, being ignorant of the righteousness of God, and seeking to establish their own, they did not submit to God's righteousness' (Romans 10:3). 'Christ is the end of the Law for righteousness to everyone who believes' (Romans 10:4). And how could Christ be the Law's end if He were not also its beginning? The One who brought the end also wrought the beginning. He is the One who said to Moses, 'I have surely seen the affliction of my people in Egypt, and I have come down to deliver them' (Exodus 3:7–8). From the start, the Word of God has been accustomed to ascend and descend for the salvation of the afflicted.

Since the Law had already taught that man must follow Christ, Jesus makes this plain to the man who asked what he must do to inherit eternal life. 'If you would enter life, keep the commandments,' He replied. When the man asked, 'Which ones?' Jesus listed them: 'You shall not commit adultery, you shall not murder, you shall not steal, you shall not bear false witness, honor your father and mother, and love your neighbor as yourself' (Matthew 19:17–19) — offering the Law's commandments as steps toward life for all who would follow Him. What He said to one, He said to all. The man claimed, 'I have kept all these' (though perhaps he had not, or Jesus would not have told him to keep the commandments). Exposing his covetousness, the Lord said, 'If you would be perfect, go, sell what you possess, give to the poor, and come, follow me' (Matthew 19:21), promising a share in the apostles' lot to those who do so. He did not proclaim a different God the Father to His followers than the One announced from the beginning by the Law; nor a different Son; nor a 'mother' or the thirty Aeons, shown to be empty and unstable; nor the fable concocted by other heretics. Instead, He taught them to do the commandments God gave from the beginning, to undo their ancient covetousness through good works, and to follow Christ. Zacchaeus demonstrated that distributing possessions to the poor satisfies the debt of past covetousness: 'Behold, Lord, half of my goods I give to the poor, and if I have defrauded anyone, I restore fourfold' (Luke 19:8).