@servanna : In the end, we're not supposed to judge the teacher, but rather discern the message.
Whether or not CS Lewis was a Christian, good Christian, bad Christian is not for us to judge. We are, however, supposed to discern any spiritual messages we hear and test them against Scripture.
But never are we given the permission to judge people. Each person has their own responsibility to choose what they will "eat" (listen to) and what they won't. If you choose to eat poison, then you will end up sick. If you refuse the poison, you will remain healthy.
If you find someone to be a heretic or an apostate, just move on. Bashing people doesn't do any good. I see far too much infighting and bashing within the Body of Christ these days, and it really needs to stop. Let the Lord fight His own battles, our job is to reach souls and teach them the truth.