One stands for God. It is the basis of theism. Jews contended, often with considerable argument from heathen neighbors, that there was only one god and any additional gods is fraud. In actual practice, worshiping more than one god is confusing and a bit futile. Where do you turn next? There is supposed to be up to 3 million gods in India. Why bother.
The expansive world of demons and other wicked beings believe in one god.
James 2:19 Thou believest that there is one God; thou doest well: the devils also believe, and tremble.
A modern translation would be – You believe in one god – big deal. This is a good thing to remember when people announce they believe in God. They say something like this, “Well, I believe in God.” It may well show that is the minimal extent of their knowledge. If so, you may have a chance to present the gospel.
Keep in mind that Muslims believe in Allah but this leader of the faith comes from long held beliefs in a war god or moon god, going back many generations. Islam also thinks the Christian Trinity is 3 gods which it is not. (See symbolism number 3.) There is internet stuff on this and modern Israelis hotly contend that the word Allah is never used to denote the Old Testament God Yahweh. The notion that all roads lead to the true God is clearly amiss. Public and private behavior tells you about yourself and others. An evil person serves an evil deity. Could not be simpler.