Bin Laden's letter to America.

[Nic Name]

in·fi·del

n.
An unbeliever with respect to a particular religion, especially Christianity or Islam.
One who has no religious beliefs.
One who doubts or rejects a particular doctrine, system, or principle.

Isn't the idea of a translation to reflect the commonly understood language of the reader?

The Arabic word isn't infidel, it's a word in another language that means infidel or disbeliever. Take your pick.

It seems that folks view "authentic" as something that bin Laden would have written in English. How would he phrase it in English? What English words would he use? Well, it was written in Arabic and the translator is free to choose the proper literate, rather than literal, translation.