Who was not before Adam. The Bible does state in Genesis that God said lets us make man in OUR image. The Christian believes that "OUR image" refers to God the Father, God the Son, and God the Holy Spirit. Now if we take it back just to Noah and his family and we believe that these 8 were the only ones left. From these 8 the earth was repopulated. As seen from Adam to Noah. Adam knew God and walked with Him in the garden. Adam's kin throgh generations didn't retain God in their knowledge. This lead to eventual worship of other gods. Then the flood came and brought us to Noah. The same thing reguarding knowledge of God happened with the generations after Noah.(including modern man) They began to worship other gods. Through out all this time, God has never let certain groups forget how He really is. This is done in order to keep His worship true. Remember in the 10 Commandments? I the Lord thy God am a jealous God. Here is where Satan takes things and preverts beliefs, thus you have different gods worshipped, name one here.(Islam, Buddah, etc...)
The Christian believes that there will be a 7 year tribulation period befor Christ's return. During this period there will be a man of peace revieled. For 3-1/2 years he will make peace on earth. During the second 3-1/2 years he will set himself up as god in the temple.
To believe as we do about other gods and why they are worshipped, one must look at Satan and his purpose, (and the aforementioned things), to be god. Once this is realized, a Christian beliefs about why there are other religions beside Judism and Christianity, should become clear and can be answered in one word,Satan.