CS Lewis stated that hell is something where people deliberately put themselves in. He does have an idea of how frightening it is but at other times he could either be projecting his fears onto others (same with me and other Christians in general) or that he found it much too frightening to admit and reflect on. He does have faith in God but it seemed in his case, he’s too afraid to fully explain the nature of hell and evil (the very things other Christians are willing to do in full detail.)
He’s not any less of a Christian but one who has a lot more difficulty admitting the full nature of hell and evil. It’s not necessarily always related to his interest in pagan folklore. But also a matter of him being at other times much too disturbed to admit what he saw and reflect on it at times. He possibly knows how scary it is but he also doesn’t want to admit how damning it can be at times. He’s not really a traitor.
But he does seem very timid to the idea of suffering and sin sometimes.