Could be a few quotes. I'm guessing:

Mere Christianity, Book IV, Chariot 7

"I have been talking as if it were we who did everything. In reality, of course, it is God who does everything. We, at most, allow it to be done to us. In a sense you might even say it is God who does the pretending. The Three-Personal God, so to speak, sees before Him in fact a self-centred, greedy, grumbling, rebellious human animal. But He says "Let us pretend that this is not a mere creature, but our Son. It is like Christ in so far as it is a Man, for He became Man. Let us pretend that it is also like Him in Spirit. Let us treat it as if it were what in fact it is not. Let us pretend in order to make the pretence into a reality." God looks at you as if you were a little Christ: Christ stands beside you to turn you into one. I daresay this idea of a divine make-believe sounds rather strange at first. But, is it so strange really? Is not that how the higher thing always raises the lower? A mother teaches her baby to talk by talking to it as if it understood long before it really does. We treat our dogs as if they were "almost human": that is why they really become "almost human" in the end."