What do you think is the atheists strongest argument against Christianity?

I am a Christian, but I have my opinion as to what the strongest argument might be that would hinder an atheist from becoming a believer, but I was wanting some input from others.  Please keep this civil and please don't give a flood of different arguments.  Just give what you think is the ONE most compelling... show more I am a Christian, but I have my opinion as to what the strongest argument might be that would hinder an atheist from becoming a believer, but I was wanting some input from others.  Please keep this civil and please don't give a flood of different arguments.  Just give what you think is the ONE most compelling argument.  (For example, if you think it is an alleged contradiction in the scriptures, just give the ONE that you feel is the MOST compelling.)  Actually, as a Christian, I have never heard an argument against Christianity that couldn't be answered and that hasn't been answered many times in the past, but perhaps you have one I haven't heard before.  Again, let's don't argue!  I'm just wondering what prevents others from believing.  (Also please identify if you are a Christian, atheist, agnostic, Jew, or other religious persuasion.)
Update: While there is not as many specific answers/examples as I had hoped, there seems to be several categories that appear fairly often.  These include (in no particular order) Unanswered prayers or human suffering People who have not seen, have not seen, do not recognize or deny the existence available evidence. ... show more While there is not as many specific answers/examples as I had hoped, there seems to be several categories that appear fairly often. 

These include (in no particular order)

Unanswered prayers or human suffering

People who have not seen, have not seen, do not recognize or deny the existence available evidence.  (Although most in this group seem to have not looked for any.)

Alleged Bible contradictions (but I have had a hard time getting anyone to tell me the one they find most compelling).
Update 2: Division among “Christians”.  Jesus recognized this problem and said that unity would help others to believe (John 17:20-21) So many “Fake Christians” or false gods. But just because there are counterfeits, that does not mean the real thing does not exist.  If someone had 10 pieces of money and 9 are counterfeit,... show more Division among “Christians”.  Jesus recognized this problem and said that unity would help others to believe (John 17:20-21)

So many “Fake Christians” or false gods. But just because there are counterfeits, that does not mean the real thing does not exist.  If someone had 10 pieces of money and 9 are counterfeit, does that mean the real one does not exist?

Of course, some gave the “I’m right because Christianity BS”, but that really offers nothing to support their view.
Update 3: Some said the answers to objections offered have been invalid, but gave few (if any) examples.  Of course, it could be that the objection was also an invalid objection to begin with. Anyway, I am thankful that most of the answers were civil.  We can discuss these matters without being mean or nasty!  In fact, we... show more Some said the answers to objections offered have been invalid, but gave few (if any) examples.  Of course, it could be that the objection was also an invalid objection to begin with.

Anyway, I am thankful that most of the answers were civil.  We can discuss these matters without being mean or nasty!  In fact, we should do so.

I purposefully chose a "best answer" that presented a view I do not agree with, but they did give examples, which many did not. 
78 answers 78