My argument against God was that the universe seemed so cruel and unjust. But how had I got this idea of just and unjust? A man does not call a line crooked unless he has some idea of a straight line. What was I comparing this universe with when I called it unjust? If the whole show was bad and senseless from A to Z, so to speak, why did I, who was supposed to be part of the show, find myself in such violent reaction against it? A man feels wet when he falls into water, because man is not a water animal: a fish would not feel wet. Of course I could have given up my idea of justice by saying it was nothing but a private idea of my own. But if I did that, then my argument against God collapsed too—for the argument depended on saying that the world was really unjust, not simply that it did not happen to please my fancies. Thus in the very act of trying to prove that God did not exist—in other words, that the whole of reality was senseless—I found I was forced to assume that one part of reality—namely my idea of justice—was full of sense. Consequently, atheism turns out to be too simple. If the whole universe has no meaning, we should never have found out that it has no meaning: just as, if there were no light in the universe and therefore no creatures with eyes, we should never know it was dark. Dark would be a word without meaning.
Quotes from Mere Christianity, Part 15
C.S. Lewis, Mere Christianity (1952; Harper Collins: 2001) 38-39.
adamlickey.com 
C. S. Lewis has confused a great number of things here. In particular, he has confused his sense of justice for justice. Your sense of justice (or sense of morality, for that matter) doesn’t necessarily reflect true justice.
Take, for example, a rich man who got rich through inheritance, that feels it is just for him to get a tax cut. After all, it’s his money. Compare that to a poor person that feels justice would mean they get a tax cut, because they rely on every penny just to survive, and that the rich man should pay more tax because his good fortune should help the less well off. Both of these people feel they are right.
But the fact of the matter is that there is an option that will make the most people happy and make the fewest people sad. That is the moral option. Your feelings have nothing to do with it. And that fact is not authored, it emerges.
The problem is that these arguments, the ones that pin things like morality and justice on God, are arguments that refuse to define the concepts. What is morality? Vague answers like “what one ought to do” don’t help. Specific answers, like “actions that concern themselves with the wellbeing of conscious creatures” (to approximately quote Sam Harris) do help.
If my latter answer is what morality is, then it is not authored; it appears out of the fact that we interact and can experience.
Both “justice” and “meaning” are concepts that have their roots in human thought and action. Neither of them apply to the universe as a whole. The universe is neither just, nor unjust–neither meaningful in itself, nor meaningless–it just IS. (Calling the universe “just” or “unjust” is similar to calling a rock “evil.”)
I discuss justice in conjunction with the concept of “fairness” here: On Fairness and Justice.