Why Virtues and Not Obedient Acts?

From C.S. Lewis' Mere Christianity:
“There is a difference between doing some particular just or temperate action and being a just or temperate man. Someone who is not a good tennis player may now and then make a good shot. What you mean by a good tennis player is a man whose eye and muscles and nerves have been so trained by making innumerable good shots that they can now be relied on. They have a certain tone or quality which is there even when he is not playing, just as a mathematician’s mind has a certain habit and outlook which is there even when he is not doing mathematics. In the same way a man who perseveres in doing just actions gets in the end a certain quality of character. Now it is that quality rather than the particular actions which we mean when we talk of a “virtue”.

This disctinction is important for the following reason. If we thought only of the particular actions we might encourage three wrong ideas:

1. We might think that, provided you did the right thing, it did not matter how or why you did it—whether you did it sulkily or cheerfully, through fear of public opinion or for its own sake. But the truth is that right actions done for the wrong reason do not help to buile the internal quality or character called a ‘virtue’, and it is this quality or character that really matters.
(If the bad tennis player hits very hard, not because he sees that a very hard stroke is required, but because he lost his temper, his stroke might possibly, by luck, help him to win that particular game; but it will not be helping him to become a reliable tennis player.)

2. We might think that God wanted simply obedience to a set of rules; whereas He really wants people of a particular sort.

3. We might think that the ‘virtues’ were necessary only for this present life—that in the other world we could stop being just because there is nothing to quarrel about and stop being brave because there is no danger. Now it is quite true that there will probably be no occasion for just or courageous acts in the next world, but there will be every occasion for being the sort of people that we can become only as the result of doing such acts here.

The point is not that God will refuse you admission to His eternal world if you have not got certain qualities of character: the point is that if people have not got at least the beginnings of those qualities inside them, then no possible external conditions could make a ‘Heaven’ for them—that is, could make them happy with the deep, strong, unshakable kind of happiness God intends for us."