Just because the Bible was written after Jesus (as we know it) dont meant that paganism was the original belief system.
Honestly I dont understand why people that obviously have never thought much about religion even answer these kinds of questions, but I see it quite a bit here at "answers"- but thats beside the point, sorry.
To clear something else up, the old testament books were all written before Jesus- since the first 5 books were written by moses.
To answer the question though- the reason paganism isnt the original belief system is this;
God created man- God had a personal relationship with Man and communicated with man from the beginning, either personally or through angels. After the original sin, mankind lost much of his contact with God, and just like today- people that cant communicate or hear God, look for things to believe in- things they can see, like the sun, moon and stars. Then some strange thing would happen coincidentally and then they start believing in rocks and cows and things like that. The further people get from God, the more they try to discover Him- then at some point, anything they can draw comfort from becomes a god to them.
Adam and Eve, Satan, Moses, Noah- all believed in God, they all had personal contact with God.
Something to think about is this, even though Satan wishes NO ONE believed in God- even Satan knows He exists.