8 Now mark what is said in Proverbs: “The wise man is fixed like the sun; but the fool changes like the moon.” And who is the wise that has no changes, but that Sun of Righteousness of whom it is said, “The Sun of righteousness has risen upon me,” and of which the wicked shall say, when mourning in the day of judgment that it has not risen upon them, “The light of righteousness has not shone upon us, and the sun has not risen upon us”? For that sun which is visible to the eye of sense, God makes to rise upon the evil and the good alike, as He sends rain upon the just and the unjust; but apt similitudes are often borrowed from things visible to explain things invisible. Again, who is the “fool” who “changes like the moon,” if not Adam, in whom all have sinned? For the soul of man, receding from the Sun of righteousness, that is to say, from the internal contemplation of unchangeable truth, turns all its strength towards external things, and becomes more and more darkened in its deeper and nobler powers; but when the soul begins to return to that unchangeable wisdom, the more it draws near to it with pious desire, the more does the outward man perish, but the inward man is renewed day by day, and all that light of the soul which was inclining to things that are beneath is turned to the things that are above, and is thus withdrawn from the things of earth; so that it dies more and more to this world, and its life is hid with Christ in God.
9. It is therefore for the worse that the soul is changed when it moves in the direction of external things, and throws aside that which pertains to the inner life; and to the earth, i.e. to those who mind earthly things, the soul looks better in such a case, for by them the wicked is commended for his heart's desire, and the unrighteous is blessed. But it is for the better that the soul is changed, when it gradually turns away its aims and ambition from earthly things, which appear important in this world, and directs them to things nobler and unseen; and to the earth, i.e. to men who mind earthly things, the soul in such a case seems worse. Hence those wicked men who at last shall in vain repent of their sins, will say this among other things: “These are the men whom once we derided and reproached; we in our folly esteemed their way of life to be madness.” Now the Holy Spirit, drawing a comparison from things visible to things invisible, from things corporeal to spiritual mysteries, has been pleased to appoint that the feast symbolic of the passing from the old life to the new, which is signified by the name Pascha, should be observed between the 14th and 21st days of the month—after the 14th, in order that a twofold illustration of spiritual realities might be gained, both with respect to the third epoch of the world, which is the reason of its occurrence in the third week, as I have already said, and with respect to the turning of the soul from external to internal things—a change corresponding to the change in the moon when on the wane; not later than the 21st, because of the number 7 itself, which is often used to represent the notion of the universe, and is also applied to the Church on the ground of her likeness to the universe.
Source: Letters (New Advent)