Objection 1. It would seem that confession is according to the natural law. For Adam and Cain were bound to none but the precepts of the natural law, and yet they are reproached for not confessing their sin. Therefore confession of sin is according to the natural law.
Objection 2. Further, those precepts which are common to the Old and New Law are according to the natural law. But confession was prescribed in the Old Law, as may be gathered from Isaiah 43:26: "Tell, if thou hast anything to justify thyself." Therefore it is according to the natural law.
Objection 3. Further, Job was subject only to the natural law. But he confessed his sins, as appears from his words (Job 31:33) "If, as a man, I have hid my sin." Therefore confession is according to the natural law.
On the contrary, Isidore says (Etym. v.) that the natural law is the same in all. But confession is not in all in the same way. Therefore it is not according to the natural law. Further, confession is made to one who has the keys. But the keys of the Church are not an institution of the natural law; neither, therefore, is confession.
I answer that, The sacraments are professions of faith, wherefore they ought to be proportionate to faith. Now faith surpasses the knowledge of natural reason, whose dictate is therefore surpassed by the sacraments. And since "the natural law is not begotten of opinion, but a product of a certain innate power," as Tully states (De Inv. Rhet. ii), consequently the sacraments are not part of the natural law, but of the Divine law which is above nature. This latter, however, is sometimes called natural, in so far as whatever a thing derives from its Creator is natural to it, although, properly speaking, those things are said to be natural which are caused by the principles of nature. But such things are above nature as God reserves to Himself; and these are wrought either through the agency of nature, or in the working of miracles, or in the revelation of mysteries, or in the institution of the sacraments. Hence confession, which is of sacramental necessity, is according to Divine, but not according to natural law.
Reply to Objection 1. Adam is reproached for not confessing his sin before God: because the confession which is made to God by the acknowledgment of one's sin, is according to the natural law. whereas here we are speaking of confession made to a man. We may also reply that in such a case confession of one's sin is according to the natural law, namely when one is called upon by the judge to confess in a court of law, for then the sinner should not lie by excusing or denying his sin, as Adam and Cain are blamed for doing. But confession made voluntarily to a man in order to receive from God the forgiveness of one's sins, is not according to the natural law.
Reply to Objection 2. The precepts of the natural law avail in the same way in the law of Moses and in the New Law. But although there was a kind of confession in the law of Moses, yet it was not after the same manner as in the New Law, nor as in the law of nature; for in the law of nature it was sufficient to acknowledge one's sin inwardly before God; while in the law of Moses it was necessary for a man to declare his sin by some external sign, as by making a sin-offering, whereby the fact of his having sinned became known to another man; but it was not necessary for him to make known what particular sin he had committed, or what were its circumstances, as in the New Law.
Reply to Objection 3. Job is speaking of the man who hides his sin by denying it or excusing himself when he is accused thereof, as we may gather from a gloss [Cf. Gregory, Moral. xxii, 9 on the passage.
Source: Summa Theologica (New Advent)