NOTES for Exo 20:2-3
This is the very first commandment of the Decalogue (the Ten Commandments of Moses). The sages of Israel said that the whole Law is contained in it, because all the other commandments are only a commentary, an explanation of how to fulfill this first commandment.
It may not be clear at once why this is so. In what way do the commandments "make yourself no idol," "do not kill," not to mention the prohibition against "boiling a kid in its mother's milk" (Ex. 23:19), explain the meaning and significance of the commandment of monotheism? There is a connection here, but it is not very direct.
What must first be understood is what it means for us that God exists at all. If He does not exist, then the only creator and being determining the paths of this world turns out to be man. Then man is his own authority and his own criterion of rightness. One may recall the King from Schwartz's An Ordinary Miracle: he is his own king, honorary Pope, honorary saint, and so on. Or, more seriously, Dostoevsky: "If there is no God, then everything is permitted."
Now let us see what happens if God is not one, but there are many gods. Then they inevitably turn out to be "particular," functional gods, having only a certain sphere of influence (god of the sea, god of the sun, god of rain, god of a specific tribe, and the like). And man, being a creature with universal needs, begins to "maneuver" among different gods, using their strengths and weaknesses for his own interests, thereby placing himself one step above each of these gods. But if God is one, that does not work: a universal God has no strong and weak sides between which one could "slip through." If God is one, then He is inevitably higher than man.
Why is the Exodus from Egypt mentioned in this commandment? Even if God exists and He is one, a person has two possible ways to behave, depending on whether this God is good to man or not. If He is not, then one must run as far away from such a God as possible. And if He is good (and God gives an example of His good deed for people), then one must draw near to Him, seek what He wants for us, what He considers good. And who can reveal to us what He considers good except He Himself? And then the other commandments appear, by which God describes His idea of human righteousness. We see that without accepting the first commandment, all the rest simply make no sense!