I was baptized and raised Catholic. When I was 19, I left the Catholic Church for Protestantism. At what point, if ever, does missing Mass stop being a mortal sin?
Missing Mass, without just cause, always remains grave matter. However, for there to be a mortal sin, there must also be full knowledge of the gravity of the action and full and free consent of the will to the action. If any of those conditions are missing, there is not a mortal sin. In other words, missing Mass is always a serious condition, but an individual’s personal culpability may be mitigated due to his particular level of knowledge and consent.
If you left the Church for Protestantism because you sincerely believed that by doing so you would be following God’s will, leaving the Church was objectively incorrect but you would not be personally culpable for a mortal sin because you were honestly doing your best to follow what you sincerely believed to be God’s will. Thus, you also would not be personally culpable for sin by missing Mass because you honestly believed it was God’s will that you not attend Mass.
Should you ever come to the point in your Christian journey where you receive the grace to believe again in Catholicism, you would then be responsible to attend Mass on Sundays and holy days of obligation now that you know that God wills that Christians should do so.
The Long Way Home** by Karl Keating