2 All is contained in these brief sentences: “Love the Lord your God with all your heart, and with all your soul, and with all your strength: and love your neighbour as yourself;” for these are the words in which the Lord, when on earth, gave an epitome of religion, saying in the gospel, “On these two commandments hang all the law and the prophets.” Daily advance, then, in this love, both by praying and by well-doing, that through the help of Him, who enjoined it on you, and whose gift it is, it may be nourished and increased, until, being perfected, it render you perfect. “For this is the love which,” as the apostle says, “is shed abroad in our hearts by the Holy Ghost, which is given unto us.” This is “the fulfilling of the law;” this is the same love by which faith works, of which he says again, “Neither circumcision avails anything, nor uncircumcision; but faith, which works by love.”
Source: Letters (New Advent)