As far as “a priori truth claims” – secular humanism has them as well. Let’s start with a materialistic origin to the universe. The anti-theist simply claims this to be true because they deny the existence of the metaphysical – which is itself a metaphysical claim. “We can’t conclusively prove there’s anything ‘outside’ the universe, so God can’t be there.” It’s a somewhat-rational leap if one discounts what one can’t prove empirically.