Lewis hit on a very profound point. Apart from a transcendent God what makes anything “right”, “wrong” or “evil”? Only a transcendent God can establish an objective morality or ethic. Apart from that all you’ve got is personal preference or social convention and those can and do change frequently.
‘Apart from a transcendent God what makes anything “right”, “wrong” or “evil”?’
think of whatever it is that you would not want others, in the normal course of events, to do to you, and thus refrain yourself from doing likewise to those others...’right, wrong, or evil’ is breaking that simple code...