For Lewis, It was matter of coming to think that there are a bunch of lines of evidence that you believe support theism, or even Christianity. If you read his book it's hard to escape the idea that he thought he had reasons to become a Christian, and that this made him psychologically uncomfortable. Whether these reasons were good or bad is not the point here.
People like Dawkins think that the evidence against theism is overwhelming. What if someone were to gradually move from that position to believe that the case for atheism is less than overwhelming, then about 50-50, and then it starts looking like God really does exist after all.
If that happened to someone, a lot of atheists might react the way Nagel would:
“In speaking of the fear of religion, I don’t mean to refer to the entirely reasonable hostility toward certain established religions and religious institutions, in virtue of their objectionable moral doctrines, social policies, and political influence. Nor am I referring to the association of many religious beliefs with superstition and the acceptance of evident empirical falsehoods. I am talking about something much deeper–namely, the fear of religion itself. I speak from experience, being strongly subject to this fear myself: I want atheism to be true and am made uneasy by the fact that some of the most intelligent and well-informed people I know are religious believers.
I want atheism to be true and am made uneasy by the fact that some of the most intelligent and well-informed people I know are religious believers. It isn’t just that I don’t believe in God and, naturally, hope that I’m right in my belief. It’s that I hope there is no God! I don’t want there to be a God; I don’t want the universe to be like that.”(”The Last Word” by Thomas Nagel, Oxford University Press: 1997)”
And that is how Lewis reacted when it happened to him.