Most people seem to think C.S. Lewis wrote nothing except Narnia. This isn't the case; Lewis was a convert to Christianity and wrote forcefully and critically about it, even while he subscribed to it. A good example of this is Mere Christianity.

Narnia itself was a critique: yes, Aslan is Jesus, but Lewis' point wasn't "Jesus loves you!" It was "Jesus is a force of nature and he is REALLY FUCKING SCARY."

He's not the kind of man I like defending, but Lewis wasn't doing "propaganda" any more than Dante was doing "propaganda" when he wrote Inferno. He was trying to speak truth to power and using fiction to do it.