Let me say at the beginning that The Book Of Eli is a silly and stupid movie. Save your money. The only thing worth the ticket price is Denzel Washington's portrayal of Eli.
The story is post apocalyptic with the required marauding gangs on motorcycles that pillage and rape. And of course, Eli, the man of God, kills all the bad guys with this machete, his guns, and his bow and arrow.
He is taking "the book" (the bible) west to an unknown destination because some voice told him to. He has been on this pilgrimage for 30 years. Oh yes, there is a pretty young thing, half his age, who wants to have sex with him and looks seductive and alluring enough, but Eli has the masculine discipline to refuse and she falls in love with him and follows him on his pilgrimage and takes up his cause in the end.
As a morality tale it leaves a lot to be desired. The bible didn't save us the first time, and the message of the film is that it will the second but gives no clear idea of why this would be.
Save your money, but the fundies will go nuts with its apocalyptic and biblical themes.