The reason that people have a problem with mortal sin is because they look at things from the standpoint of God punishing the sinner. That’s not really the best way to look at it, and its not entirely theologically correct either.
To make it really simple, let me explain it this way. God is infinite. Men are not. Men are not compatible with God. For a man to live in the realm of God would be like for a man to live in the ocean - we just aren’t born with what we need to do that. Now man has devised a piece of equipment to let him survive under water - scuba gear. God has also devised a sort of “equipment” to allow man to survive in Heaven - sanctifying grace. When a person is baptized, he is given the sanctifying grace, which is a bit like “God’s scuba gear.” Much like the Protestant understanding of things, God gives us this “scuba gear” totally free based on faith. Through Jesus Christ, our sins are forgiven, so He does not take away the gear everytimg we sin. That’s a good thing, because as we go through life we commit little sins all the time.
But the thing is, this divine scuba gear really is pretty bulky, and it takes up all the room there is on the soul. Just like a scuba diver can’t fit a football uniform on over his gear, we can’t fit anything else on over God’s sanctifying grace - His eternal scuba gear. But sometimes in life, some, or most, people see something they want to put on their souls. Maybe it is the overcoat of fornication, or the three piece suit of murder, or even the casual dress of missing Mass on Sunday. When we want to put these things on our soul, we have to take that scuba gear off, first.
That may sound a bit silly, and it sortof is the way I put it. BUt it makes the point. Mortal sins don’t send us to Hell so much because God is punishing us. They send us because commiting them results in us rejecting the sanctifying grace that lets us exist in Heaven. We can’t do them without rejecting that grace. People don’t understand mortal sin because they see it as God rejecting us for one act, whereas what it really is is us rejecting God. As everyone knows, it only takes rejecting God once to reject Him. This is where Catholic theology is soooo much closer to mainline Protestantism without Protestants really realizing it. They say that God gives us justification completely free, regardless of if we sin or not, by the merits of Jesus Christ. The thing is, Catholics say that too, we just recognize that we can still reject God. You can do this by literally saying it, or by doing something that conveys the same meaning. Does a husband have to actually tell his wife he is mad at her, or would, say, punching her do the same thing? Obviously our actions can convey messages. The mortal sin isn’t just in the sin - its in that the sinner is rejecting of God and His offer of salvation. It’s the act of saying, “I want this more than I want God.”
That’s another good way to look at it. Part of the problem is the term salvation. It’s a correct term, but it tends to make us look at things the wrong way by our own faults, not the fault of the word. Even St. Paul used the word, after all. But remember that Jesus didn’t really talk about salvation. He talked about inheriting eternal life, or inheriting the kingdom of God, or attaining eternal life. St. Paul did, too. This is a much better way to talk about things. Salvation carries the notion of a person being rescued from something, in this case sin and hell. This is truely what Christ does, but its more than that. God created us as free creatures. He deeply wants us to come and live with Him, but He won’t force us. He offers us heaven, and we then have to make the choice. That’s what life is all about - making that choice. As we start our Christian life, we are on the ‘yes’ side. This may be a little off color, but its the best thing I can come up with now. When two people are starting to engage in the marital act, the woman may at any time decide she doesn’t want to do it and she may ask to stop, even though she already made the really big choice in the first place. Now some men won’t listen to this, and it becomes rape. God doesn’t do this. If we decide we don’t want to be with Him, even after we’ve made the choice to at first, He stops. He doesn’t force us. He respects us. If we decide, after accepting His invitation, that we’d rather go live somewhere else, He will accept that. That is mortal sin. It is turning down God’s invitation after we have accepted it. The thing is, as we walk away from His house, God is always begging us to come back. “Please,” He says, “come in - I will really make you feel at home!” But He never forces us. If we reaccept His invitation, this is confession. It is all about our choices, not God simply punishing us.
