Cant remember book title.?

It,s a sci fi book in space.I think the description on the back, says "The ship is one mile long and has no windows.....'. The humans on "The Ship' dont remember the original mission. They think the original captain is god. There are different levels, and gravity lessens as you go up. The upper levels have mutants. The mutants know the truth of the mission, but fight with the humans. Thats all I remember.