2 I am the LORD thy God, which have brought thee out of the land of Egypt, out of the house of bondage.
3 Thou shalt have no other gods before me.
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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 immediately clear why this is so. In what way do the commandments "do not make an idol for yourself," "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.
First of all, we must understand what it means for us that God exists at all. If He does not exist, then the only creator and being who determines the paths of this world turns out to be man. Man then is his own chief and the criterion of correctness. One can recall the King from Schwartz's An Ordinary Miracle: he is his own king, honorary Pope of Rome, honorary saint, and so on. Or, more seriously, Dostoevsky: "If God does not exist, 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 "private," functional gods, having only a certain sphere of influence, the god of the sea, the god of the sun, the god of rain, the god of a particular tribe, and so on. And man, being a creature with universal needs, begins to "maneuver" among different gods, using their strengths and weaknesses in his own interests, thereby placing himself one step above each of these gods. But if God is one, this does not work: the 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, man has two paths of behavior depending on whether this God is good to man or not. If not, then one must run from such a God as far as possible. But 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 understanding of human righteousness. We see that without accepting the first commandment, all the others simply have no meaning!