If you withhold a sin in confession, are subsequent confessions invalid?
Not necessarily. If a person withholds confessing a venial sin, he would still be validly absolved. However, regarding grave/mortal sins, the Church teaches clearly that “one who desires to obtain reconciliation with God and with the Church must confess to a priest all the unconfessed grave sins he remembers after having carefully examined his conscience. The confession of venial faults, without being necessary in itself, is nevertheless strongly recommended by the Church" (CCC 1493, emphasis added; see also 1456-58).
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