I think there are two slightly different situations at work here. Lewis did grow up in a very Christian environment, and as such possessed the "antibodies" described in the linked post.

His being a convert from atheism - by means of some kind of emotional breakdown - didn't, thus, make him into an advocate of following every passage of the Bible and every idea ever presented by the Magisterium; rather, it made him a zealous defender of every excuse for why you don't need to do so in order to be a good Christian.