What movement was Chesterson writing about in "Orthodoxy"

I just started reading G.K. Chesterson's *Orthodoxy* and came across this quote: > In this remarkable situation it is plainly not now possible (with any > hope of a universal appeal) to start, as our fathers did, with the > fact of sin. This very fact which was to them (and is to me) as plain > as a pikestaff, is the very fact that has been specially diluted or > denied. But though moderns deny the existence of sin, I do not think > that they have yet denied the existence of a lunatic asylum. In context, he seems to be talking about a movement ***within Christianity***, not an atheistic movement, that denies the very existence of sin. Certainly I've heard atheists make the claim that sin is a man-made concept, and that it's "relative" - what one person considers sin may be perfectly OK for someone else, for example. But within the bounds of official Christian doctrine and understanding, the concept seems incomprehensible to me. Why would someone bother self-identifying as a Christian if they didn't need Christ's atonement for their sins? What would be the point? My own incredulity aside, there are enough beliefs within Christianity that it wouldn't surprise me to find teachings that deny the very existence of sin, but I've never really paid attention to such things, or bothered to learn about them. Now my curiosity is piqued. Was there such a movement at the time of the writing of the book, and how widespread was it? And further, is there still such a belief within recognized traditions (even those considered cults by more orthodox Christianity) that are known for teaching the idea that sin doesn't exist?