If you withhold a sin in confession, are subsequent confessions invalid?
Consequently, one who willfully withholds a mortal sin cannot be validly absolved of his sins, because he has not manifested true contrition for his grave wrongdoing (CCC 1456; see 1452-53, 1855-56, 1861). However, if in a subsequent confession that person confesses that willfully withheld mortal sin—and any other mortal sins he has committed and which he can recall—he will be validly absolved. It’s all up to him. We might be able to fool a priest, but we can’t fool the omniscient Lord.
By Tom Nash