Thomas Aquinas

“ If, however, the sin has escaped from his memory altogether, then he is excused from his duty through being unable to fulfill it, and it is enough that he be contrite in general for everything wherein he has offended God. But when this inability is removed, as when the sin is recalled to his memory, then he is bound to have contrition for that sin in particular, even as a poor man, who cannot pay a debt, is excused, and yet is bound to, as soon as he can. ”

“ Art. 5 - Whether a man ought to have contrition for another's sin? Objection 1: It would seem that a man ought to have contrition for another's sin. For one should not ask forgiveness for a sin unless one is contrite for it. ”

“ Now it is evident in the generation of a body, accompanied by corruption of another body, that all the accidents contrary to the thing generated, and which were the accidents of the thing corrupted, are removed by the one generation: whereas in alteration, only that accident is removed which was contrary to the accident which is the term of the alteration. In like manner, one Baptism blots out all sins together and introduces a new life; whereas Penance does not blot out each sin, unless it be directed to each. For this reason it is necessary to be contrite for, and to confess each sin. ”

“ Yet we do not speak of contrition for future sins, but of caution, which is a part of prudence conjoined to penance.
Reply to Objection 1: The free-will is said to regard future contingents, in so far as it is concerned with acts, but not with the object of acts: because, of his own free-will, a man can think about past and necessary things, and yet the very act of thinking, in so far as it is subject to the free-will, is a future contingent. Hence the act the contrition also is a future contingent, in so far as it is subject to the free-will ”

“ Accordingly a sin also may be forgotten in two ways, either so as to remain in a general, but not in a particular remembrance, and then a man is bound to bethink himself in order to discover the sin, because he is bound to have contrition for each individual mortal sin. ”

“ Now original sin has a deformity, even as actual sin has. Therefore we should be contrite on its account also.
Objection 2: Further, by original sin man has been turned away from God, since in punishment thereof he was to be deprived of seeing God. But every man should be displeased at having been turned away from God. Therefore man should be displeased at original sin; and so he ought to have contrition for it. ”

“ The prophet prays to be spared from the sins of others, in so far as, through fellowship with sinners, a man contracts a stain by consenting to their sins: thus it is written (Ps. 17:27) : "With the perverse thou wilt be perverted." ”