Objection 1. It would seem that there was no cause for the ceremonial precepts. Because on Ephesians 2:15, "Making void the law of the commandments," the gloss says, (i.e.) "making void the Old Law as to the carnal observances, by substituting decrees, i.e. evangelical precepts, which are based on reason." But if the observances of the Old Law were based on reason, it would have been useless to void them by the reasonable decrees of the New Law. Therefore there was no reason for the ceremonial observances of the Old Law.
Objection 2. Further, the Old Law succeeded the law of nature. But in the law of nature there was a precept for which there was no reason save that man's obedience might be tested; as Augustine says (Gen. ad lit. viii, 6,13), concerning the prohibition about the tree of life. Therefore in the Old Law there should have been some precepts for the purpose of testing man's obedience, having no reason in themselves.
Objection 3. Further, man's works are called moral according as they proceed from reason. If therefore there is any reason for the ceremonial precepts, they would not differ from the moral precepts. It seems therefore that there was no cause for the ceremonial precepts: for the reason of a precept is taken from some cause.
On the contrary, It is written (Psalm 18:9): "The commandment of the Lord is lightsome, enlightening the eyes." But the ceremonial precepts are commandments of God. Therefore they are lightsome: and yet they would not be so, if they had no reasonable cause. Therefore the ceremonial precepts have a reasonable cause.
I answer that, Since, according to the Philosopher (Metaph. i, 2), it is the function of a "wise man to do everything in order," those things which proceed from the Divine wisdom must needs be well ordered, as the Apostle states (Romans 13:1). Now there are two conditions required for things to be well ordered. First, that they be ordained to their due end, which is the principle of the whole order in matters of action: since those things that happen by chance outside the intention of the end, or which are not done seriously but for fun, are said to be inordinate. Secondly, that which is done in view of the end should be proportionate to the end. From this it follows that the reason for whatever conduces to the end is taken from the end: thus the reason for the disposition of a saw is taken from cutting, which is its end, as stated in Phys. ii, 9. Now it is evident that the ceremonial precepts, like all the other precepts of the Law, were institutions of Divine wisdom: hence it is written (Deuteronomy 4:6): "This is your wisdom and understanding in the sight of nations." Consequently we must needs say that the ceremonial precepts were ordained to a certain end, wherefrom their reasonable causes can be gathered.
Reply to Objection 1. It may be said there was no reason for the observances of the Old Law, in the sense that there was no reason in the very nature of the thing done: for instance that a garment should not be made of wool and linen. But there could be a reason for them in relation to something else: namely, in so far as something was signified or excluded thereby. On the other hand, the decrees of the New Law, which refer chiefly to faith and the love of God, are reasonable from the very nature of the act.
Reply to Objection 2. The reason for the prohibition concerning the tree of knowledge of good and evil was not that this tree was naturally evil: and yet this prohibition was reasonable in its relation to something else, in as much as it signified something. And so also the ceremonial precepts of the Old Law were reasonable on account of their relation to something else.
Reply to Objection 3. The moral precepts in their very nature have reasonable causes: as for instance, "Thou shalt not kill, Thou shalt not steal." But the ceremonial precepts have a reasonable cause in their relation to something else, as stated above.
Source: Summa Theologica (New Advent)