I had always thought that the 1st Commandment was the way that monotheists differentiated themselves from the multitheists of ancient Egypt (for Hebrews), and later of Rome (for Christians). I didn't ever think that it was a way of saying, "We're Right (c), and the rest of you can go pound sand."
Weren't the Children of Isreal down in the valley building (multitheic) idols to worship while Moses was on his mountain retreat?