You shall have no other gods before Me. You shall not make for yourself a graven image, nor any manner of likeness of any thing that is in heaven above, that is in the earth beneath, or that is in the water under the earth. You shall not bow down to them, nor serve them. For I the Lord your God am a jealous God, visiting the iniquity of the fathers upon the children of the third and fourth generation of those that hate Me; and showing mercy unto the thousandth generation of those that love Me and keep My commandments (Exodus 20:3-5).
Prohibiting the worship of any other god seems like an obvious next step after the First Commandment (“I am the Lord your God”), and the instruction to have no other gods seems fairly obvious. As basic as this commandment may seem, the text itself is very specific, detailing both ambiguous worship (unseen other gods) and the worship of any natural part of the universe (in heaven above, earth below or in the water–i.e. sun, moon, stars, etc). Since the worship of idols, according to the Midrash, stemmed from people thinking of the sun or thunder as more approachable servants of God, these details are important.
In addition to the detailed definition of not having other gods, the text of the Second Commandment includes what seems like an explanation of the reason not to worship other gods. The second half of the Second Commandment, which includes details such as God being a jealous god, conveys, in human terms, the seriousness of this transgression. Because God loves the Jewish people, He notifies them of the dire consequences of idol worship; that it doesn’t just affect one person, but impacts on many generations.