Irenaeus — Against Heresies 4.12

It clearly appears that there was but one author of both the old and the new law, from the fact that Christ condemned traditions and customs repugnant to the former, while He confirmed its most important precepts, and taught that He was Himself the end of the Mosaic law.

1. For the tradition of the elders themselves, which they pretended to observe from the law, was contrary to the law given by Moses . Wherefore also Esaias declares: Your dealers mix the wine with water, Isaiah 1:22 showing that the elders were in the habit of mingling a watered tradition with the simple command of God ; that is, they set up a spurious law, and one contrary to the [ true ] law; as also the Lord made plain, when He said to them, Why do you transgress the commandment of God , for the sake of your tradition? Matthew 15:3 For not only by actual transgression did they set the law of God at nought, mingling the wine with water; but they also set up their own law in opposition to it, which is termed, even to the present day, the pharisaical. In this [law] they suppress certain things, add others, and interpret others, again, as they think proper, which their teachers use, each one in particular; and desiring to uphold these traditions, they were unwilling to be subject to the law of God , which prepares them for the coming of Christ . But they did even blame the Lord for healing on the Sabbath-days, which, as I have already observed, the law did not prohibit. For they did themselves, in one sense, perform acts of healing upon the Sabbath day , when they circumcised a man [on that day]; but they did not blame themselves for transgressing the command of God through tradition and the aforesaid pharisaical law, and for not keeping the commandment of the law, which is…

2. But that this is the first and greatest commandment, and that the next [has respect to love ] towards our neighbour, the Lord has taught, when He says that the entire law and the prophets hang upon these two commandments. Moreover, He did not Himself bring down [from heaven] any other commandment greater than this one, but renewed this very same one to His disciples , when He enjoined them to love God with all their heart, and others as themselves. But if He had descended from another Father, He never would have made use of the first and greatest commandment of the law; but He would undoubtedly have endeavoured by all means to bring down a greater one than this from the perfect Father, so as not to make use of that which had been given by the God of the law. And Paul in like manner declares, Love is the fulfilling of the law: Romans 13:10 and [he declares] that when all other things have been destroyed, there shall remain faith , hope, and love ; but the greatest of all is love ; 1 Corinthians 13:13 and that apart from the love of God , neither knowledge avails anything, 1 Corinthians 13:2 nor the understanding of mysteries , nor faith , nor prophecy , but that without love all are hollow and vain; moreover, that love makes man perfect; and that he who loves God is perfect, both in this world and in that which is to come. For we do never cease from loving God ; but in proportion as we continue to contemplate Him, so much the more do we love Him.

3. As in the law, therefore, and in the Gospel [likewise], the first and greatest commandment is, to love the Lord God with the whole heart, and then there follows a commandment like to it, to love one's neighbour as one's self; the author of the law and the Gospel is shown to be one and the same. 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.