Objection 1. It would seem that the ceremonies of the Law were in existence before the Law. For sacrifices and holocausts were ceremonies of the Old Law, as stated above (Question 101, Article 4). But sacrifices and holocausts preceded the Law: for it is written (Genesis 4:3-4) that "Cain offered, of the fruits of the earth, gifts to the Lord," and that "Abel offered of the firstlings of his flock, and of their fat." Noah also "offered holocausts" to the Lord (Genesis 18:20), and Abraham did in like manner (Genesis 22:13). Therefore the ceremonies of the Old Law preceded the Law.
Objection 2. Further, the erecting and consecrating of the altar were part of the ceremonies relating to holy things. But these preceded the Law. For we read (Genesis 13:18) that "Abraham... built... an altar to the Lord"; and (Genesis 28:18) that "Jacob... took the stone... and set it up for a title, pouring oil upon the top of it." Therefore the legal ceremonies preceded the Law.
Objection 3. Further, the first of the legal sacraments seems to have been circumcision. But circumcision preceded the Law, as appears from Genesis 17. In like manner the priesthood preceded the Law; for it is written (Genesis 14:18) that "Melchisedech... was the priest of the most high God." Therefore the sacramental ceremonies preceded the Law.
Objection 4. Further, the distinction of clean from unclean animals belongs to the ceremonies of observances, as stated above (100, 2, 6, ad 1). But this distinction preceded the Law; for it is written (Genesis 7:2-3): "Of all clean beasts take seven and seven... but of the beasts that are unclean, two and two." Therefore the legal ceremonies preceded the Law.
On the contrary, It is written (Deuteronomy 6:1): "These are the precepts and ceremonies... which the Lord your God commanded that I should teach you." But they would not have needed to be taught about these things, if the aforesaid ceremonies had been already in existence. Therefore the legal ceremonies did not precede the Law.
I answer that, As is clear from what has been said (101, 2; 102, 2), the legal ceremonies were ordained for a double purpose; the worship of God, and the foreshadowing of Christ. Now whoever worships God must needs worship Him by means of certain fixed things pertaining to external worship. But the fixing of the divine worship belongs to the ceremonies; just as the determining of our relations with our neighbor is a matter determined by the judicial precepts, as stated above (Question 99, Article 4). Consequently, as among men in general there were certain judicial precepts, not indeed established by Divine authority, but ordained by human reason; so also there were some ceremonies fixed, not by the authority of any law, but according to the will and devotion of those that worship God. Since, however, even before the Law some of the leading men were gifted with the spirit of prophecy, it is to be believed that a heavenly instinct, like a private law, prompted them to worship God in a certain definite way, which would be both in keeping with the interior worship, and a suitable token of Christ's mysteries, which were foreshadowed also by other things that they did, according to 1 Corinthians 10:11: "All... things happened to them in figure." Therefore there were some ceremonies before the Law, but they were not legal ceremonies, because they were not as yet established by legislation.
Reply to Objection 1. The patriarchs offered up these oblations, sacrifices and holocausts previously to the Law, out of a certain devotion of their own will, according as it seemed proper to them to offer up in honor of God those things which they had received from Him, and thus to testify that they worshipped God Who is the beginning and end of all.
Reply to Objection 2. They also established certain sacred things, because they thought that the honor due to God demanded that certain places should be set apart from others for the purpose of divine worship.
Reply to Objection 3. The sacrament of circumcision was established by command of God before the Law. Hence it cannot be called a sacrament of the Law as though it were an institution of the Law, but only as an observance included in the Law. Hence Our Lord said (John 7:20) that circumcision was "not of Moses, but of his fathers." Again, among those who worshipped God, the priesthood was in existence before the Law by human appointment, for the Law allotted the priestly dignity to the firstborn.
Reply to Objection 4. The distinction of clean from unclean animals was in vogue before the Law, not with regard to eating them, since it is written (Genesis 9:3): "Everything that moveth and liveth shall be meat for you": but only as to the offering of sacrifices because they used only certain animals for that purpose. If, however, they did make any distinction in regard to eating; it was not that it was considered illegal to eat such animals, since this was not forbidden by any law, but from dislike or custom: thus even now we see that certain foods are looked upon with disgust in some countries, while people partake of them in others.
Source: Summa Theologica (New Advent)