Both words have so many levels of value that ambiguity is unavoidable. But I'll argue that spiritual has a less defined and unconventional (non-Abrahamic) connotation, while the term ' religious ' refers to an Judeo-Christian or Abraham-based belief system where God has a personality that hears prayers and may respond. Thus, in a spiritual system, one (probably) does not believe in a personality that hears prayers an thus, grace has no meaning. Fundamentally, prayer is superstitious wishing, and all non-scientific belief is superstitious in nature. Thus, grace may well be given up by those like the label of spiritual, but it's still all superstition, whether spiritual or religious.

That's optional.