The proof is simple. We can divide all ethical statements into three categories:
- Ethical beliefs that correspond to deep biological and psychological characteristics of human beings
- Ethical beliefs (economic relations) directly impelled by the means of production and sensitively dependent on the specific means of production
- Everything else
Economic relations, however deep their justification in scripture, don't survive substantive changes in the means of production; when the means of production change, the religious themselves give up these ethical beliefs. One need only look at the Abrahamic religions' justification for slavery: once slave-based economic relations were superseded by industrialization, the (successful) religions and sects "reinterpreted" their scripture to condemn slavery.
So that leaves the third category. Lacking a naturalistic or economic justification, these beliefs are therefore arbitrary, even on the theistic account of moral ontology: they are subject to the will and judgment of the deity, by definition an essentially personal being. (Deism, pantheism, and panentheism make no ethical claims, and its unclear that they make any interesting ontological claims.)
It's well-established in sociology and anthropology that arbitrary socially-constructed beliefs serve to define an in-group. Assuming that both the in- and out-groups are naturalistically adapted (a prohibition against capricious killing doesn't serve well to define an in-group), it is precisely the arbitrariness of these beliefs that allow them to serve as a group definition. It's unlikely that anyone outside the group would adopt the belief by chance; if it were adopted by chance it would not be preserved and reinforced by physical reality.
Hence, all interesting religious ethical beliefs are arbitrary. Q.E.D.