In Judaism, the Sh'ma states "The Lord is our God; the Lord is one." In Islam, the Shahada states "There is no God but Allah and Muhammed is His Prophet." Both of these religions go out of their way to explicitly state that the God they worship is the only real one. The thing is, one might expect that to be a given -- it seems like any religion would deny the existence of other gods. But this rejection is extremely important to both religions (the Sh'ma and the Shahada are very important prayers and the idea is found in other places as well). It makes me wonder if, for example, the Hittites thought the Egyptian gods probably existed but chose not to worship them, and monotheistic traditions like Judaism and Islam reject other gods' existence so explicitly because it was not the norm for religions to do so when they were founded. Is this true at all? If not, why is it so prominent in Judaism and Islam and not in other modern religions?

Of course, Islam and Judaism were formed centuries apart and in vastly different cultural contexts (not least because Christianity came about some time in between). I'd welcome an answer on either one, both, or even just on various ancient religions' beliefs about each other (e.g. Greeks and Persians or Romans and Gauls).