Spiritual sloth is inattention to one's own soul. It is quite compatible with being a busybody and attending to other people's souls (in an unhelpful way): as Neko Case laments of her small-town neighbors in one of her songs '. . .mind all of my business, and none of their own. They're all so happy now that I done wrong, I'm surprised they don't come over to thank me.'
Oh yeah, even I get that ;) . That's why adduced my Baptist pastor friend. It seems to me any activity, from rosaries to Bible reading to giving one's body to be burned, could coexist with interior sloth. But as attempt at an argument a posteriori that double predestination and "blesses assurance" lead to sloth will need some evidence, I think. And I'm not seeing any. And an a priori argument will ..., well, let's stand back and watch. I bet the fur will fly.