If muawiyah is not being facetious up to this point, then can see why some atheists would have a big chip on their shoulders toward Christians and Christianity—not all Christians are so belligerent towards atheists, or other non-Christians.
If someone "pledges" within the framework of a known, well-practiced, worked-out religious standard, we have a way of judging them.
BTW, the Yakuza request knuckle pledges from their subordinates for much the same reason. I thought it might well be applicable here.
So, absent religious belief I'll accept a pound of flesh as a surity.
Makes me quite liberal when you get down to it.