New Study: atheists are less open-minded than theists

[A Theist]

I wonder how we atheists would be treated on these Christian message boards.

Why don't you join one and find out. If you don't go on their sites looking for a fight and start calling everyone there Nazis, racists, bigots, like some here on the forum do to people they disagree with, you might be well received.
Although, I don't believe atheism has anything to do with whether or not a person is either open minded or close minded. I think that has more to do with which political side a person embraces. Left wingers are far more hateful, authoritarian, and much less tolerant of opposing opinions than those on the right.