Is Moral Nihilism a Morality?

[Alan V]

I fail to see how theistic belief can serve as a basis for morality. 

I am better acquainted with Sufi interpretations of Islam that with Christianity. According to what I learned, the basic principle is "no god but God," which due to the names of God also translates to "no absolute but the Absolute." This introduces a kind of iconoclastic, relativistic morality to at least a part of the Islamic community rather early on, and lays the basis for any number of moral arguments against any possible kind of idolatry. So it provides the rationale for a kind of virtue ethics, where specific virtues can be supported by their relationship to avoiding idolatry of worldly things. Religious rituals of various sorts can also be rationalized by this underlying monotheistic principle.