Is missing Mass on Sunday a mortal sin? You have to stop and define mortal sin. First of all, a mortal sin is something which, by its very nature orients us away from God rather than toward God. One of the ten commandments is ‘keep holy the Sabbath Day’. How do you sanctify the Sabbath? By worshipping God. How do you worship God? By going to Mass. If you don’t to this you orient your life away from God and if you continue in this same direction then you will not end up with God. You will end up away from God, and since God is love and light and beauty and truth, then you will end up without love and light and beauty and truth, and that means death. So is missing Mass a mortal sin? Yes.