6 How, then, is work perfected in the number forty? The reason, it may be, is, because the law was given in ten precepts, and was to be preached throughout the whole world: which whole world, we are to mark, is made up of four quarters, east and west, south and north, whence the number ten, multiplied by four, comes to forty. Or, it may be, because the law is fulfilled by the gospel, which has four books: for in the gospel it is said, “I came not to destroy the law, but to fulfill it.” Whether, then, it be for this reason or for that, or for some other more probable, which is hid from us, but not from more learned men; certain it is, however, that in the number forty a certain perfection in good works is signified, which good works are most of all practised by a kind of abstinence from unlawful lusts of the world, that is, by fasting in the general sense.
Hear also the apostle when he says, “Love is the fulfilling of the law.” Whence the love? By the grace of God, by the Holy Spirit. For we could not have it from ourselves, as if making it for ourselves. It is the gift of God, and a great gift it is: for, says he, “the love of God is shed abroad in our hearts by the Holy Spirit, which is given to us.” Wherefore love completes the law, and most truly it is said, “Love is the perfecting of the law.” Let us inquire as to this love, in what manner the Lord does commend it to our consideration. Remember what I laid down: I want to explain the number thirty-eight of the years of that impotent man, why that number thirty-eight is one of weakness rather than of health. Now, as I was saying, love fulfills the law. The number forty belongs to the perfecting of the law in all works; but in love two precepts are committed to our keeping. Keep before your eyes, I beseech you, and fix in your memory, what I say; be ye not despisers of the word, that your soul may not become a trodden path, where the seed cast cannot sprout, “and the fowls of the air will come and gather it up.” Apprehend it, and lay it up in your hearts. The precepts of love, given to us by the Lord, are two: “You shall love the Lord your God with all your heart, and with all your soul, and with all your mind;” and, “You shall love your neighbor as yourself. On these two commandments hang all the law and the prophets.” With good reason did the widow cast “two mites,” all her substance, into the offerings of God: with good reason did the host take “two” pieces of money, for the poor man that was wounded by the robbers, for his making whole: with good reason did Jesus spent two days with the Samaritans, to establish them in love. Thus, while a certain good thing is generally signified by this number two, most especially is love in its twofold character set forth to us thereby. If, therefore, the number forty possesses the perfecting of the law, and the law is fulfilled only in the twin precepts of love, why do you wonder that he was weak and sick, who was short of forty by two?
Source: Tractates on the Gospel of John (New Advent)