Doesn't matter; to some around here, anyone not from north west Europe is merely a Muslim in hiding, and obviously a terrorist lying in wait.
Unfortunately true.
Given his Nigerian roots, and the presence in that nation of Muslims bent on imposing Sharia law, it is a perfectly fair question to ask whether this fellow is Muslim or not.
As for the name "Joseph" -- it's possible that he selected it as his "American name." I've known a lot of folks from different countries (mainly Asian) who've done that.
Given the preponderance of Muslims among our terrorist foes, this guy's religious affiliation (if any) would appear to be relevant to the story.