You will have no other gods besides Me.
The First Commandment can be said to be the hook, designed to get our attention. No other Gods. period. What about that do we not understand?
Modes and his homeboys certainly understood. They'd just come from Egypt, after all, where just about everything could be deified. The Egyptians had jackal-headed gods, and cow-headed gods, and gods that looked like storks. It was pretty confusing to the average Egyptian, who generally picked one god to worship in the house...or hut, temple. (This kind of 'selection' is common to most polytheistic cultures.)
Even so, there were a number of Israelites who would have been just as happy hedging their bets with other gods. Maybe the God of Israel wasn't quite strong enough in some areas...let's give Him some backup!
Hedging bets. That's really they key, isn't it?
The First Commandment really says, "count on me...you don't need anyone else".
In the 21st century, we don't have to much trouble with golden calves or gods with weird heads and weirder names. But we do have problems with hedging bets.
Such as - "he who dies with the most toys wins".
In other words...this might be all we got, this life. So grab what you can, and live life to the 'fullest'.
Who knows if you'll really get pie in the sky when you die?
God's saying, BELIEVE! It's such an important concept that He made it a Commandment.
The first one.