As I understand it, the essence of Ethical Monotheism is that morality must be absolute, thus it requires a supranatural (i.e. external to the bounds of our existence) basis, i.e. a God.
Unfortunately I can't (yet) find any rational support for this thesis.
Respecting the origin of the Universe three verbally intelligible suppositions may be made. We may assert that it is self-existent; or that it is self-created; or that it is created by an external agency. The deeper question, into which this finally merges, is,
whether any one of them is even conceivable in the true sense of the word.
Whoever agrees that the atheistic hypothesis is untenable because it involves the impossible idea of self-existence, must perforce admit that the theistic hypothesis is untenable if it contains the same impossible idea.
It is not a question of probability, or credibility, but of conceivability. Experiment proves that the elements of these hypotheses cannot even be put together in consciousness.
Differing so widely as they seem to do, the atheistic, the pantheistic, and the theistic hypotheses contain the same ultimate element.
It is impossible to avoid making the assumption of self-existence somewhere; and whether that assumption be made nakedly or under complicated disguises, it is equally vicious, equally unthinkable.http://www.constitution.org/hs/first_prin.htm