To be obstinate is to be stubborn, and the tendency of humankind toward God is extreme stubbornness. C.S. Lewis phrases it this way: "The Son of God became a man to enable men to become sons of God.... The natural life in each of us is something self-centered, something that wants to be petted and admired, to take advantage of other lives ... and especially it wants to be left to itself: to keep well away from anything better or stronger or higher than it, anything that might make it feel small.... It knows that if the spiritual life gets hold of it, all its self-centeredness and self-will are going to be killed and it is ready to fight tooth and nail to avoid that.... Imagine turning a tin soldier into a real little man. It would involve turning the tin into flesh. And suppose the tin soldier did not like it. He is not interested in flesh: all he sees is that the tin is being spoiled. He thinks you are killing him. He will do everything he can to prevent you."
"What you would have done about that tin soldier I do not know. But what God did about us is this: The Second Person in God, the Son, became human Himself ... a real man of a particular height, with hair of a particular color, speaking a particular language.... The Eternal Being, who knows everything and who created the whole universe, became not only a man but (before that) a baby.... Think how you would like to become a slug or a crab. The result of this was that you now had one man who really was what all men were intended to be: one man in whom the created life ... allowed itself to be completely and perfectly turned into the begotten life.... Because the whole difficulty for us is that the natural life has to be, in a sense, 'killed,' He chose an earthly career that involved the killing of His human desires at every turn—poverty, misunderstanding from His own family, betrayal by one of His intimate friends, being jeered at and manhandled by the Police, and execution by torture. And then ... the Man in Christ rose again: not only the God. That is the whole point. For the first time we saw a real man. One tin soldier—real tin, just like the rest, had come fully and splendidly alive."
"We have not got to try to climb up into spiritual life by our own efforts; it has already come down into the human race. If we will only lay ourselves open to the one new man in whom it was fully present ... He will do it in us and for us. Remember what I said about 'good infection.' One of our own race has this new life: if we get close to Him, we shall catch it from Him. Of course, you can express this in all sorts of ... ways. You can say that Christ died for our sins. You may say that the Father has forgiven us because Christ has done for us what we ought to have done. You may say that we are washed in the blood of the Lamb. You may say that Christ has defeated death. They are all true."
Highlights from Chapter 5: The Obstinate Toy Soldiers from Book 4: Beyond Personality, or First Steps in the Doctrine of the Trinity in Mere Christianity by C.S. Lewis. Click here for a clear view of how this chapter relates to the whole book.