18 Nevertheless the seventh day was appointed to the Jewish nation as a day to be observed by rest of the body, that it might be a type of sanctification to which men attain through rest in the Holy Spirit. We do not read of sanctification in the history given in Genesis of all the earlier days: of the Sabbath alone it is said that “God blessed the seventh day, and sanctified it.” Now the souls of men, whether good or bad, love rest, but how to attain to that which they love is to the greater part unknown: and that which bodies seek for their weight, is precisely what souls seek for their love, namely, a resting-place. For as, according to its specific gravity, a body descends or rises until it reaches a place where it can rest—oil, for example, falling if poured into the air, but rising if poured into water—so the soul of man struggles towards the things which it loves, in order that, by reaching them, it may rest. There are indeed many things which please the soul through the body, but its rest in these is not eternal, nor even long continued; and therefore they rather debase the soul and weigh it down, so as to be a drag upon that pure imponderability by which it tends towards higher things. When the soul finds pleasure from itself, it is not yet seeking delight in that which is unchangeable; and therefore it is still proud, because it is giving to itself the highest place, whereas God is higher. In such sin the soul is not left unpunished, for “God resists the proud, but gives grace to the humble.” When, however, the soul delights in God, there it finds the true, sure, and eternal rest, which in all other objects was sought in vain. Therefore the admonition is given in the book of Psalms, “Delight yourself in the Lord, and He shall give you the desires of your heart.”
19. Because, therefore, “the love of God is shed abroad in our hearts by the Holy Spirit which is given to us,” sanctification was associated with the seventh day, the day in which rest was enjoined. But inasmuch as we neither are able to do any good work, except as helped by the gift of God, as the apostle says, “For it is God that works in you both to will and to do of His good pleasure,” nor will be able to rest, after all the good works which engage us in this life, except as sanctified and perfected by the same gift to eternity; for this reason it is said of God Himself, that when He had made all things “very good,” He rested “on the seventh day from all His works which He had made.” For He, in so doing, presented a type of that future rest which He purposed to bestow on us men after our good works are done. For as in our good works He is said to work in us, by whose gift we are enabled to work what is good, so in our rest He is said to rest by whose gift we rest.
Source: Letters (New Advent)