12 We are therefore bound to denounce with abhorrence and contempt the ravings of the astrologers, who, when we find fault with the empty inventions by which they cast other men down into the delusions where into they themselves have fallen, imagine that they answer well when they say, “Why, then, do you regulate the time of the observance of Easter by calculation of the positions of the sun and moon?”— as if that with which we find fault was the arrangements of the heavenly bodies, or the succession of the seasons, which are appointed by God in His infinite power and goodness, and not their perversity in abusing, for the support of the most absurd opinions, those things which God has ordered in perfect wisdom. If the astrologer may on this ground forbid us from drawing comparisons from the heavenly bodies for the mystical representation of sacramental realities, then the augurs may with equal reason prevent the use of these words of Scripture, “Be harmless as doves;” and the snake-charmers may forbid that other exhortation, “Be wise as serpents;” while the play-actors may interfere with our mentioning the harp in the book of Psalms. Let them therefore say, if they please, that, because similitudes for the exhibition of the mysteries of God's word are taken from the things which I have named, we are chargeable either with consulting the omens given by the flight of birds, or with concocting the poisons of the charmer, or with taking pleasure in the excesses of the theatre—a statement which would be the clime of absurdity.
13. We do not forecast the issues of our enterprises by studying the sun and moon, and the times of the year or of the month, lest in the most trying emergencies of life, we, being dashed against the rocks of a wretched bondage, shall make shipwreck of our freedom of will; but with the most pious devoutness of spirit, we accept similitudes adapted to the illustration of holy things, which these heavenly bodies furnish, just as from all other works of creation, the winds, the sea, the land, birds, fishes, cattle, trees, men, etc., we borrow in our discourses manifold figures; and in the celebration of sacraments, the very few things which the comparative liberty of the Christian dispensation has prescribed, such as water, bread, wine, and oil. Under the bondage, however, of the ancient dispensation many rites were prescribed, which are made known to us only for our instruction as to their meaning. We do not now observe years, and months, and seasons, lest the words of the apostle apply to us, “I am afraid of you, lest I have bestowed upon you labour in vain.” For he blames those who say, “I will not set out today, because it is an unlucky day, or because the moon is so and so;” or, “I will go today, that things may prosper with me, because the position of the stars is this or that; I will do no business this month, because a particular star rules it;” or, “I will do business, because another star has succeeded in its place; I will not plant a vineyard this year, because it is leap year.” No man of ordinary sense would, however, suppose that those men deserve reproof for studying the seasons, who say, e.g., “I will not set out today, because a storm has begun;” or, “I will not put to sea, because the winter is not yet past;” or, “It is time to sow my seed, for the earth has been saturated with the showers of autumn;” and so on, in regard to any other natural effects of the motion and moisture of the atmosphere which have been observed in connection with that consummately ordered revolution of the heavenly bodies concerning which it was said when they were made, “Let them be for signs, and for seasons, and for days, and for years.” And in like manner, whenever illustrative symbols are borrowed, for the declaration of spiritual mysteries, from created things, not only from the heaven and its orbs, but also from meaner creatures, this is done to give to the doctrine of salvation an eloquence adapted to raise the affections of those who receive it from things seen, corporeal and temporal, to things unseen, spiritual and eternal.
Source: Letters (New Advent)