People often think of Christian morality as a kind of bargain in which God says, “If you keep a lot of rules I’ll reward you, and if you don’t I’ll do the other thing.” I do not think that is the best way of looking at it. I would much rather say that every time you make a choice you are turning the central part of you, the part that chooses, into something a little different from what it was before. And taking your life as a whole, with all your innumerable choices, all your life long you are slowly turning this central thing either into a heavenly creature or into a hellish creature: either into a creature that is in harmony with God, and with its fellow-creatures, and with itself. To be one kind of creature is heaven: that is, it is joy and peace and knowledge and power. To be the other means madness, horror, idiocy, rage impotence and eternal loneliness. Each of us at each moment is progressing to the one state or to the other.Then she says:
That explains what always used to puzzle me about Christian writers; they seem to be so very strict at one moment and so very free and easy at another. They talk about mere sins of thought as if they were immensely important: and then they talk about the most frightful murders and treacheries as if you had only got to repent and all would be forgiven. But I have come to see that they are right. What they are always thinking of is the mark which the action leaves on that tiny central self which no one sees in this life but which each of us will have to endure—or enjoy—for ever.
To me, this seems to be a particularly Catholic way of thinking--Catholics, feel free to argue--and though I have no intention of converting, it makes a lot of sense to me. If you’re going to be stuck with ‘you’ forever, it behooves you to try to become the type of person that you want to be stuck with for that long. After your threescore and ten are done (plus change, if you’re lucky/unlucky), all opportunities for modification will be past. To put it in geek-speak, in the eternal life realm, you'll have all the features and bugs that you’re ever going to have. Apart from radical change, a person is always headed in one direction or the other; therefore the time to make changes is now.
I am not a Catholic, but I am going to challenge that reading of the passage. I don't think Lewis is saying we are going to bear the marks of sin forever. We are going to be perfected in Christ. Rather Lewis is speaking directly of people Now, the eternal beings we meet in the grocery store. For a Christian the passage to becoming more like Jesus is increasingly difficult as you further your walk with Him. The more He sheds his light on your life more more you become aware of where the dross is. Some things are easy to get rid of.
Imagine the sin in your life as a big log. Jesus helps you roll the huge log off your chest, suddenly you're free, you can move, He's bandaging your big scrapes and cuts. You notice you are covered in splinters, some are big - easy to see, most of them are tiny hard - to grasp. All of the splinters are areas that can cause infection. All need to be dealt with. So large unmistakeable sins, like murder to use Lewis' example, can not be ignored, it is easy to let the Physician pull the large shards. Sins of thought, are tiny things requiring us to search carefully, get it all including the sharp spike; to stand under Jesus' strong searching light and not flinch away from His tweezers.