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:” 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;” and that apart from the love of God, neither knowledge avails anything, 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.
Source: Adversus haereses (New Advent)