The command Be ye perfect is not idealistic gas. Nor is it a command to do the impossible. He is going to make us into creatures that can obey that command. He said (in the Bible) that we were ‘gods’ and He is going to make good His words. If we let Him—for we can prevent Him, if we choose—He will make the feeblest and filthiest of us into a god or goddess, a dazzling, radiant, immortal creature, pulsating all through with such energy and joy and wisdom and love as we cannot now imagine, a bright stainless mirror which reflects back to God perfectly (though, of course, on a smaller scale) His own boundless power and delight and goodness. The process will be long and in parts very painful, but that is what we are in for. Nothing less. He meant what He said.
From Mere Christianity
Compiled in A Year with C.S. Lewis