The second Mitzva of the Ten Commandments says:
"You shalt not have any other gods besides Me."
All other nations and civilizations in antiquity, conceived the existence and coexistence of several gods. They saw in nature (and in human society) forces that opposed to each other. Life and death, light and darkness, good and evil, etc. Their reasoning was that multiple gods must be responsible for the diversity and opposite powers of these forces.
We Jews believe that there are no powers independent from, or beyond God's control. Abraham Abinu's idea of One (and invisible) God, revolutionized humanity in many ways. Not just as an arithmetic reduction of gods, but mainly because of the political and moral implications of it.
Let me explain: Conceiving the existence of more than one God, represents an open invitation for moral relativism. One day this god is worshiped by waging war. The next day we worship the god of peace. Next week the god of love. This week the god of murder. One god is worshipped by being completely drunk. The other by not touching alcohol. Nothing is morally right or wrong in itself and forever. The morality of one's actions depends on the god one would worship that day. Which will be different from the god we will worship the following day. And who establishes which god we should worship each day? Well, that is the prerogative of the King and his entourage of priests and ministers. The effects of idol-worshipping were well beyond the theological aspect of it. As Maimonides explained, ido-worshipping served the political power and it established a society of relative morality.
The belief in one God to the exclusion of all sort of powers means, among other things, moral clarity. The existence of one absolute and unchangeable set of moral values. Coming from one God.
Jewish monotheism leaves no room for moral blurriness.