Judaism and the seven laws of Noah?

I asked before about the topic but I am really confuse, according to rabbi and master rambam the christianity and islam are part of the God in order to spread the God's law because they claim to worship the God of Abraham.. but according to the first command it says not worship false gods.. or idolatry.. then other religions are not Ok, once a rabbi I asked him and he told me they have the obligation to discover those laws but why?