Love Commended by Our Lord Himself
Moreover, the Lord Jesus Himself teaches us that the whole law and the prophets hang upon the two precepts of love to God and love to our neighbour. Concerning these two commandments the following is written in the Gospel according to St. Mark: “And one of the scribes came, and having heard them reasoning together, and perceiving that He had answered them well, asked Him: Which is the first commandment of all? And Jesus answered him: The first of all the commandments is, Hear, O Israel! The Lord our God is one Lord; and you shall love the Lord your God with all your heart, and with all your soul, and with all your mind, and with all your strength. This is the first commandment. And the second is like it: You shall love your neighbour as yourself. There is none other commandment greater than these.” Also, in the Gospel according to St. John, He says, “A new commandment I give unto you, that you love one another; as I have loved you, that you also love one another. By this shall all men know that you are my disciples, if you have love to one another.”
Source: On Grace and Free Will (New Advent)