The hostility can also be based upon doubt.
Religion is about convincing yourself that God exists. But everyone is fallible and makes mistakes. What if theists have made a mistake about their belief? Going to houses of worship, studying holy books and praying won't make that doubt go away. It remains there because it is impossible to convince yourself to the point of removing all self doubt. In order for that to happen it must be stepped up to the level of brainwashing but that is not a conviction either.
So for theists who are honest with themselves there is always this nagging doubt and their hostility towards atheists stems from seeing their own doubts in the flesh, so to speak. The religion of theists claims that heathens suffer from all kinds of imaginary problems but in the flesh atheists are normal people leading normal lives. They are friendly, funny, open and giving. They care about others, make donations to good causes, volunteer to help the less fortunate and generally lead good lives.
And they doing all of this without any imaginary deity in their lives. They don't pray, congregate or proselytize. They don't pay tithes or claim that gay marriage will bring about the apocalypse.
Hostility towards atheists is not because they don't believe. Hostility towards atheists is because believers have a hard time dealing with their own self doubts when faced with the normality that is everyday atheism in my opinion.
It has nothing to do with a lack of faith.
The hostility may have something to do with the way all it takes is one Atheist to complain about something to have it removed. Atheists try to infringe on everyone's freedom of religious expression. I don't believe in the Easter Bunny, but that doesn't mean I'm going to sue if anyone has an Easter Egg hunt. For years Atheists tried to remove the cross on the top of Mount Soledad in San Diego.

Mount Soledad cross controversy - Wikipedia the free encyclopedia
The hostility is based in the feeling that you should live and let live. Quit wearing your feelings on your sleeves people. Stop horning in on everyone's happiness.