Objection 1. It would seem that the nature of the ceremonial precepts does not consist in their pertaining to the worship of God. Because, in the Old Law, the Jews were given certain precepts about abstinence from food (Leviticus 11); and about refraining from certain kinds of clothes, e.g. (Leviticus 19:19): "Thou shalt not wear a garment that is woven of two sorts"; and again (Numbers 15:38): "To make to themselves fringes in the corners of their garments." But these are not moral precepts; since they do not remain in the New Law. Nor are they judicial precepts; since they do not pertain to the pronouncing of judgment between man and man. Therefore they are ceremonial precepts. Yet they seem in no way to pertain to the worship of God. Therefore the nature of the ceremonial precepts does not consist in their pertaining to Divine worship.
Objection 2. Further, some state that the ceremonial precepts are those which pertain to solemnities; as though they were so called from the "cerei" [candles] which are lit up on those occasions. But many other things besides solemnities pertain to the worship of God. Therefore it does not seem that the ceremonial precepts are so called from their pertaining to the Divine worship.
Objection 3. Further, some say that the ceremonial precepts are patterns, i.e. rules, of salvation: because the Greek chaire is the same as the Latin "salve." But all the precepts of the Law are rules of salvation, and not only those that pertain to the worship of God. Therefore not only those precepts which pertain to Divine worship are called ceremonial.
Objection 4. Further, Rabbi Moses says (Doct. Perplex. iii) that the ceremonial precepts are those for which there is no evident reason. But there is evident reason for many things pertaining to the worship of God; such as the observance of the Sabbath, the feasts of the Passover and of the Tabernacles, and many other things, the reason for which is set down in the Law. Therefore the ceremonial precepts are not those which pertain to the worship of God.
On the contrary, It is written (Exodus 18:19-20): "Be thou to the people in those things that pertain to God... and... shew the people the ceremonies and the manner of worshipping."
I answer that, As stated above (Question 99, Article 4), the ceremonial precepts are determinations of the moral precepts whereby man is directed to God, just as the judicial precepts are determinations of the moral precepts whereby he is directed to his neighbor. Now man is directed to God by the worship due to Him. Wherefore those precepts are properly called ceremonial, which pertain to the Divine worship. The reason for their being so called was given above (Question 99, Article 3), when we established the distinction between the ceremonial and the other precepts.
Reply to Objection 1. The Divine worship includes not only sacrifices and the like, which seem to be directed to God immediately, but also those things whereby His worshippers are duly prepared to worship Him: thus too in other matters, whatever is preparatory to the end comes under the science whose object is the end. Accordingly those precepts of the Law which regard the clothing and food of God's worshippers, and other such matters, pertain to a certain preparation of the ministers, with the view of fitting them for the Divine worship: just as those who administer to a king make use of certain special observances. Consequently such are contained under the ceremonial precepts.
Reply to Objection 2. The alleged explanation of the name does not seem very probable: especially as the Law does not contain many instances of the lighting of candles in solemnities; since, even the lamps of the Candlestick were furnished with "oil of olives," as stated in Leviticus 24:2. Nevertheless we may say that all things pertaining to the Divine worship were more carefully observed on solemn festivals: so that all ceremonial precepts may be included under the observance of solemnities.
Reply to Objection 3. Neither does this explanation of the name appear to be very much to the point, since the word "ceremony" is not Greek but Latin. We may say, however, that, since man's salvation is from God, those precepts above all seem to be rules of salvation, which direct man to God: and accordingly those which refer to Divine worship are called ceremonial precepts.
Reply to Objection 4. This explanation of the ceremonial precepts has a certain amount of probability: not that they are called ceremonial precisely because there is no evident reason for them; this is a kind of
consequence. For, since the precepts referring to the Divine worship must needs be figurative, as we shall state further on (2), the consequence is that the reason for them is not so very evident.
Source: Summa Theologica (New Advent)