Question:
Answer:
Confession does indeed forgive all sins. However, if someone deliberately withholds a mortal sin from confession, it is not forgiven. If you’ve forgotten a mortal sin, then you are “conditionally” forgiven for it; you need only to confess it next time you go to confession, but you are not considered to be in a state of mortal sin. As the Catechism notes:
When Christ’s faithful strive to confess all the sins that they can remember, they undoubtedly place all of them before the divine mercy for pardon. But those who fail to do so and knowingly withhold some, place nothing before the divine goodness for remission through the mediation of the priest (1456).
A person who has genuinely forgotten a mortal sin need not worry because he is not considered in a state of mortal sin. However, once it is remembered, the mortal sin does need to be confessed, preferably before so much time has passed that there is the danger of forgetting it again.