From a letter written to his brother dated February 18, 1940. Lewis was clearly not a fan of Karl Barth’s.
Even its theology–for that is a most distressing discovery I have been making these last two terms as I have been getting to know more and more of the Christian element in Oxford. Did you fondly believe–as I did–that where you got among Christians, there, at least, you would escape (as behind a wall from a keen wind) from the horrible ferocity and grimness of modern thought? Not a bit of it. I blundered into it all, imagining that I was the upholder of the old, stern doctrines against modern quasi-Christian slush: only to find that my ’sternness’ was their ’slush’. They’ve all been reading a dreadful man called Karl Barth, who seems the right opposite number to Karl Marx. ‘Under judgment’ is their great expression. They all talk like Covenanters or Old Testament prophets. They don’t think human reason or human conscience of any value at all: they maintain, as stoutly as Calvin, that there’s no reason why God’s dealings should appear just (let alone, merciful) to us: and they maintain the doctrine that all our righteousness is filthy rags with a fierceness and sincerity which is like a blow in the face.