Baptists should use the word scripture uses: carnal. Fleshly.
And they do, as well. Backsliding is an Old Testament term which might or might not be literally apropos as it describes the behavior of a group, not of an individual, without explaining how it got that way (shift in individuals’ behavior? shift in membership?) but it tells more than “carnal” does. I don’t get bent out of shape at this terminology, I know what they mean and it’s clear enough.