/summa/TP/TPIII/TP_Q88:
We must now consider the return of sins which have been taken away by Penance: under which head there are four points of inquiry:
(1) Whether sins which have been taken away by Penance return simply through a subsequent sin?
(2) Whether more specially as regards certain sins they return, in a way, on account of ingratitude?
(3) Whether the debt of punishment remains the same for sins thus returned?
(4) Whether this ingratitude, on account of which sins return, is a special sin?