Thanks very much to the commenters for the response to my "Defending St Alphonsus" post. A couple of questions have come up about mortal sin and hell. Let's have a look at hell first...
Far from being the mark of a vindictive and malicious God who doesn't care about us, hell is something that follows as common sense from what God has done for us in his infinite love. First he has created us free and immortal. As human people, we are made to his image such that we are able to love freely and for ever. By revelation we know that God has made us to love him freely and for ever and to receive his infinite and supremely benign love for all eternity.
It is easy to see that just as the infinite love of God for all eternity is our highest good, so the loss of this love is the greatest possible misfortune. If it were possible for someone else to take it away from us, we would rightly be in anguish and rage at the utter injustice of it. Thank God, this cannot happen: God is infinitely just. But we can take his love and throw it away ourselves by deliberately sinning against him. The folly and madness of voluntarily incurring this eternal deprivation is the chief pain of the damned (the pain of loss.)
It is entirely rational and prudent to have a holy fear of this loss. A husband who loves his wife but has done some stupid thing that threatens his marriage will be sensible if he fears to lose his wife and resolves with the utmost determination never to do that stupid thing again. His fear will be an entirely balanced and sensible reaction to the situation.
If on the other hand, he is living a good life generally and is meeting the obligations of his marriage, it would be damaging to the relationship if he were living in constant anxiety about losing his wife. He might occasionally think of such a fear if some "near miss" accident happens, for example, but generally, it is better for him to be finding ways to show genuine, practical love for his family by the good things that he does from day to day.
In our relationship with God, if we have committed some sin that is grave matter and therefore possibly a "mortal sin", we are right to be fearful, to make a good confession as soon as possible, and to make a firm purpose of amendment. To carry on as though we had not a care in the world and excuse ourselves would be like the man who tells his wife "get over it" or "boys will be boys". If a man said that, you would not think that he cared much about saving his marriage. If a sinner behaves in a similarly blasé manner, he clearly doesn't understand the danger of losing God for all eternity and how awful that would be.
By our prayer, penance and reception of the sacraments, we should get into the state of not committing any sins that are grave matter but living from day to day trying to overcome our venial faults, trying to pray better and living a life of practical charity towards others. Now and again, some bad thought or other temptation can remind us of the real danger of falling into sin and how we need to take the means to guard against dangerous occasions.
(I know that there are one or two questions about "mortal sin" and I'll look at those soon.)