Objection 1. It seems that the rite of circumcision was unfitting. For circumcision, as stated above (1,2), was a profession of faith. But faith is in the apprehensive power, whose operations appear mostly in the head. Therefore the sign of circumcision should have been conferred on the head rather than on the virile member.
Objection 2. Further, in the sacraments we make use of such things as are in more frequent use; for instance, water, which is used for washing, and bread, which we use for nourishment. But, in cutting, we use an iron knife more commonly than a stone knife. Therefore circumcision should not have been performed with a stone knife.
Objection 3. Further, just as Baptism was instituted as a remedy against original sin, so also was circumcision, as Bede says (Hom. in Circum.). But now Baptism is not put off until the eighth day, lest children should be in danger of loss on account of original sin, if they should die before being baptized. On the other hand, sometimes Baptism is put off until after the eighth day. Therefore the eighth day should not have been fixed for circumcision, but this day should have been anticipated, just as sometimes it was deferred.
On the contrary, The aforesaid rite of circumcision is fixed by a gloss on Romans 4:11: "And he received the sign of circumcision."
I answer that, As stated above (Article 2), circumcision was established, as a sign of faith, by God "of" Whose "wisdom there is no number" (Psalm 146:5). Now to determine suitable signs is a work of wisdom. Consequently, it must be allowed that the rite of circumcision was fitting.
Reply to Objection 1. It was fitting for circumcision to be performed on the virile member. First, because it was a sign of that faith whereby Abraham believed that Christ would be born of his seed. Secondly, because it was to be a remedy against original sin, which is contracted through the act of generation. Thirdly, because it was ordained as a remedy for carnal concupiscence, which thrives principally in those members, by reason of the abundance of venereal pleasure.
Reply to Objection 2. A stone knife was not essential to circumcision. Wherefore we do not find that an instrument of this description is required by any divine precept; nor did the Jews, as a rule, make use of such a knife for circumcision; indeed, neither do they now. Nevertheless, certain well-known circumcisions are related as having been performed with a stone knife, thus (Exodus 4:25) we read that "Sephora took a very sharp stone and circumcised the foreskin of her son," and (Joshua 5:2): "Make thee knives of stone, and circumcise the second time the children of Israel." Which signified that spiritual circumcision would be done by Christ, of Whom it is written (1 Corinthians 10:4): "Now the rock was Christ."
Reply to Objection 3. The eighth day was fixed for circumcision: first, because of the mystery; since, Christ, by taking away from the elect, not only guilt but also all penalties, will perfect the spiritual circumcision, in the eighth age (which is the age of those that rise again), as it were, on the eighth day. Secondly, on account of the tenderness of the infant before the eighth day. Wherefore even in regard to other animals it is prescribed (Leviticus 22:27): "When a bullock, or a sheep, or a goat, is brought forth, they shall be seven days under the udder of their dam: but the eighth day and thenceforth, they may be offered to the Lord."
Moreover, the eighth day was necessary for the fulfilment of the precept; so that, to wit, those who delayed beyond the eighth day, sinned, even though it were the sabbath, according to John 7:23: "(If) a man receives circumcision on the sabbath-day, that the Law of Moses may not be broken." But it was not necessary for the validity of the sacrament: because if anyone delayed beyond the eighth day, they could be circumcised afterwards.
Some also say that in imminent danger of death, it was allowable to anticipate the eighth day. But this cannot be proved either from the authority of Scripture or from the custom of the Jews. Wherefore it is better to say with Hugh of St. Victor (De Sacram. i) that the eighth day was never anticipated for any motive, however urgent. Hence on Proverbs 4:3: "I was... an only son in the sight of my mother," a gloss says, that Bersabee's other baby boy did not count because through dying before the eighth day it received no name; and consequently neither was it circumcised.
Source: Summa Theologica (New Advent)