This is from CS Lewis’ “Mere Christianity” from the chapter on “The Three Parts of Morality”.
Again, Christianity asserts that every individual human being is going to live for ever, and this must be either true or false. Now there are a good many things which would not be worth bothering about if I were going to live only seventy years, but which I had better bother about very seriously if I am going to live for ever. Perhaps my bad temper or my jealousy are gradually getting worse so gradually that the increase in seventy years will not be very noticeable. But it might be absolute hell in a million years: in fact, if Christianity is true, Hell is the precisely correct technical term for what it would be. And immortality makes this other difference, which, by the by, has a connection with the difference between totalitarianism and democracy. If individuals live only seventy years, then a state, or a nation, or a civilisation, which may last for a thousand years, is more important than an individual. But if Christianity is true, then the individual is not only more important but incomparably more important, for he is everlasting and the life of a state or a civilisation, compared with his, is only a moment.
And that leads on to my second point. 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 of you 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 other creatures, and with itself, or else into one that is in a state of war and hatred with. God, and with its fellow-creatures, and with itself. To be the 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 the other.
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. One man may be so placed that his anger sheds the blood of thousands, and another so placed that however angry he gets he will only be laughed at. But the little mark on the soul may be much the same in both. Each has done something to himself which, unless he repents, will make it harder for him to keep out of the rage next time he is tempted, and will make the rage worse when he does fall into it. Each of them, if he seriously turns to God, can have that twist in the central man straightened out again: each is, in the long run, doomed if he will not. The bigness or smallness of the thing, seen from the outside, is not what really matters.