To "soft soap" someone is to flatter him. Here Lewis uses the term to refer to loose, sentimental talk about God that has no basis in Scripture.
In C. S. Lewis: A Biography, A. N. Wilson chides the author for using such terms: "The language and idiom of the broadcasts has dated: ‘There has been a great deal of soft soap talked about God for the last hundred years. That is not what I am offering. You can cut all that out’ . . . or ‘I personally think that next to Christianity Dualism is the manliest and most sensible creed on the market’ [p. 42]. It is hard to read these sentences without a smile." (180-81)