C.S. Lewis says that there are no good or bad impulses, but there are good or bad reactions to an impulse. You have two impulses when you see a drowning man crying for help: to run to safety, and to help the man. Which instinct is stronger is usually the one you follow. The Moral Law helps us to increase the strength of the impulse to save the man, rather than running away.
I do not quite agree with Lewis on this in the sense that there are no good or bad impulses. If we have an impulse of committing murder it is wrong. This excludes killing someone in war, because that is not murder. Most of the sins we commit, occur in our thoughts and impulses. Lust is a thought and impulse we all have at one point or another, and it is sinful thought.
C.S. Lewis says that all Christians essentially believe the same basic doctrines with many minor differences. He tells us to look at what we agree on so that we can work together as a unified Church as opposed to a bickering Church. I agree with C.S. Lewis in a sense. It is true that the Church should not bicker much among themselves, because the world watches our every move and they glorify in our disunity. However, we must be careful lest we allow heresy to creep into the Church. Some of these small things that the denominations bicker about may lead to a bigger problem. Once you start making certain concessions in the church, you may not be able to see where you draw the line. As we recall in The Screwtape Letters, the devil works slowly, by gradually causing small changes that can in the end lead to bigger changes that can destroy the purity of the Church.