That’s what I tried to answer in post 63

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Attending Sunday Mass is very important to our salvation. Knowing this, the Church has required Catholics to attend on that day, for their own good, using the authority of binding that Christ gave her. The Church speaks with the authority of Christ the King. The bishops are His representatives on Earth.
If a man knows that the Church has authoritatively required Catholics to attend Mass each Sunday, and he knows what this authority is and means, then by refusing, he is rejecting the command of Christ. This is putting his will above the will of God in a matter of grave importance to salvation, with full knowledge of what he is doing.
Being inside the Kingdom of God is being
united with God. Rebelling against God’s commands is separating oneself from God. So by putting his will above the will of God, the man separates himself from the Kingdom and places himself and his temporary desires above God and His eternal desires for the man’s good.
So the man has damned himself unless he truly, from his heart, repents.
A lifetime of good doesn’t make up for one mortal sin because of the nature of that sin. However, I will add that I suspect many Catholics who would do such a thing are probably not fully aware of how serious what they are doing is, so they may not have “full knowledge” and may not be guilty of mortal sin.
Someone who goes to a ball game instead of Mass without knowing how serious that is may very well not go to Hell. He may not have full knowledge of the Church’s teachings on the matter.
But a mortal sin is always enough to destroy a person, no matter how much good the person did before, if it really, truly is mortal. For a sin to be truly mortal, it must be concerning a very serious issue, it must be committed with full knowledge of the Church’s teachings and of how wrong is one’s action, and it must be committed freely. I think most people who would choose a ball game above obedience to the Church probably don’t have full knowledge of the Church’s teachings, but if they do, they are separating themselves from God by choosing rebellion against His authority. For when they reject the Church’s authority, they are rejecting God’s authority, and when they put themselves above the Church, they are putting themselves above God.