20 This, moreover, is the reason why the law of the Sabbath is placed third among the three commandments of the Decalogue which declare our duty to God (for the other seven relate to our neighbour, that is, to man; the whole law hanging on these two commandments). The first commandment, in which we are forbidden to worship any likeness of God made by human contrivance, we are to understand as referring to the Father: this prohibition being made, not because God has no image, but because no image of Him but that One which is the same with Himself, ought to be worshipped; and this One not in His stead, but along with Him. Then, because a creature is mutable, and therefore it is said, “The whole creation is subject to vanity,” since the nature of the whole is manifested also in any part of it, lest any one should think that the Son of God, the Word by whom all things were made, is a creature, the second commandment is, “You shall not take the name of the Lord your God in vain.” And because God sanctified the seventh day, on which He rested, the Holy Spirit— in whom is given to us that rest which we love everywhere, but find only in loving God, when “His love is shed abroad in us, by the Holy Ghost given unto us” — is presented to our minds in the third commandment, which was written concerning the observance of the Sabbath, not to make us suppose that we attain to rest in this present life, but that all our labours in what is good may point towards nothing else than that eternal rest. For I would specially charge you to remember the passage quoted above: “We are saved by hope; but hope that is seen is not hope.”
21. For the feeding and fanning of that ardent love by which, under a law like that of gravitation, we are borne upwards or inwards to rest, the presentation of truth by emblems has a great power: for, thus presented, things move and kindle our affection much more than if they were set forth in bald statements, not clothed with sacramental symbols. Why this should be, it is hard to say; but it is the fact that anything which we are taught by allegory or emblem affects and pleases us more, and is more highly esteemed by us, than it would be if most clearly stated in plain terms. I believe that the emotions are less easily kindled while the soul is wholly involved in earthly things; but if it be brought to those corporeal things which are emblems of spiritual things, and then taken from these to the spiritual realities which they represent, it gathers strength by the mere act of passing from the one to the other, and, like the flame of a lighted torch, is made by the motion to burn more brightly, and is carried away to rest by a more intensely glowing love.
Source: Letters (New Advent)