Why Do Theists Get So Upset By Atheists?

Would it be less offensive to the fundamentally religious if atheists said that their opinion, that there is no God, was based on pure belief with no supporting evidence whatsoever other than it is written in a book that men in dog collars say is "the truth"?

Presumably, as their opinion can now be categorised as belief rather than rational thought based on robustly tested evidence, by the theists' own unwritten rule, atheists would be immune from criticism.

UPDATE

I saw this on Twitter this morning.

It seems, even The Alpha Course is struggling to reverse the tide of Atheism...