there's a fine line between saying "you shouldn't be a materialist just because it's fashionable" and "my opponents are just materialists because it's fashionable".

If someone is a materialist just because it's fashionable, that's trouble. Lewis may be wrong on whether or not the Church is 'true,' but I don't think Lewis is wrong on calling out compartmentalization and inconsistency rather than thinking about whether or not doctrines are true or false.