A passage from C.S. Lewis will help explain, this comes from a collection of his essays called The Grand Miracle published by Ballantine Books,
“…one has the picture of a diver, stripping off garment after garment, making himself naked, then flashing for a moment in the air, and then down through the green, and warm, and sunlit water into the pitch-black, cold, freezing water, down into the mud and slime, then up again, his lungs almost bursting, back again to the green and warm and sunlit water, and then at last out into the sunshine, holding in his hand the dripping thing he went down to get.”
The diver here is God and he is coming down into the depths to get something, he came as Jesus. This “something” he grabs and rushes to the surface so it can bathe in the warm sunlight. It should be noted the “thing” that he holds is in his scarred hands. This “thing” is human nature.
Before Christ died on the cross, much was needed to consecrate a person, to make them holy. This is known as The Law, though Jesus did not come to do away with The Law, he did come to fulfill it. Christ was the perfect lamb sacrificed to God and because of this sacrifice we are made right with God. If Jesus fulfilled The Law and was the perfect sacrifice, because He was sinless, the Last Sacrifice, then the course of human history after His crucifixion has the opportunity to change.
For man to change into something holy, the Holy had to change into mere man. Because of this, our nature of sin has been conquered, sure it is still there, but now we know it and we can choose not to follow it. We can choose to follow the Christ Nature.
Romans 12:2 reads “And do not be conformed to this world, but be transformed by the renewing of your mind, so that you may prove what the will of God is, that which is good and acceptable and perfect.” -nasb version (emphasis my own). Paul is asking us to think about what Christ has done for us so that He can renew our mind, and this transformation will lead into a new nature, a Christ Nature.
Isn’t it beautiful?