n.

Consequently Penance holds the second place with regard to the state
of integrity which is bestowed and safeguarded by the aforesaid
sacraments, so that it is called metaphorically "a second plank after
shipwreck." For just as the first help for those who cross the sea is
to be safeguarded in a whole ship, while the second help when the
ship is wrecked, is to cling to a plank; so too the first help in
this life's ocean is that man safeguard his integrity, while the
second help is, if he lose his integrity through sin, that he regain
it by means of Penance.

Reply Obj. 1: To hide one's sins may happen in two ways: first, in
the very act of sinning. Now it is worse to sin in public than in
private, both because a public sinner seems to sin more from
contempt, and because by sinning he gives scandal to others.
Consequently in sin it is a kind of remedy to sin secretly, and it is
in this sense that the gloss says that "to hide one's sins is a
second plank after shipwreck"; not that it takes away sin, as Penance
does, but because it makes the sin less grievous. Secondly, one hides
one's sin previously committed, by neglecting to confess it: this is
opposed to Penance, and to hide one's sins thus is not a second
plank, but is the reverse, since it is written (Prov. 28:13): "He
that hideth his sins shall not prosper."

Reply Obj. 2: Penance cannot be called the foundation of the
spiritual edifice simply, i.e. in the first building thereof; but it
is the foundation in the second building which is accomplished by
destroying sin, because man, on his return to God, needs Penance
first. However, the Apostle is speaking there of the foundation of
spiritual doctrine. Moreover, the penance which precedes Baptism is
not the sacrament of Penance.

Reply Obj. 3: The three sacraments which precede Penance refer to the
ship in its integrity, i.e. to man's state of integrity, with regard
to which Penance is called a second plank.
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SEVENTH ARTICLE [III, Q. 84, Art. 7]

Whether This Sacrament Was Suitably Instituted in the New Law?

Objection 1: It would seem that this sacrament was unsuitably
instituted in the New Law. Because those things which belong to the
natural law need not to be instituted. Now it belongs to the natural
law that one should repent of the evil one has done: for it is
impossible to love good without grieving for its contrary. Therefore
Penance was unsuitably instituted in the New Law