If you look at the 10 commandments, one of them says (not a direct quote) "Thou shalt have no gods before me, for I am a jealous god..." That implies that there is more than one god, right? Whenever I ask a deeply religious Christian about this, they say that there is only one deity, and his name is God. Hmmmmm... Also, many hardcore Christians I know (I live in the South, there is no shortage of them) say that other religions are promoting idol-worship, even Judaism and Islam (which both recognize the same god is present in all of the Abrahamic traditions) are worshipping the exact same deity Christians are, just in Arabic and Hebrew.
You could argue that your interpretation of the first commandment is wrong. The first commandment could refer to metaphorical 'gods' like money, passion, wisdom, yourself, whatever; know what I mean? It stresses the importance of God. In the tribal context the Bible was written in, the tribes in Palestine each worshipped their own deity. The first commandment was used to bring in followers.
Judaism and Islam don't believe that Christ died for their sins, so you can see why Christians call it idol-worship. In my opinion, God reveals himself to different people and different ways. All religions are equally right and equally wrong.