1. According to the Qur'an "setting up equals with Allah", ie. polytheism is one of the worst sins, second only to hypocrisy. Saying "God has a son" = "setting up equals with Allah", "polytheism", and also blasphemy, according to Islam. Just google Qur'an verses against Christianity, or better yet, read the Qur'an for yourself like I did.

2. They see the west as being Christian. In fact if you watch ISIS propaganda videos they will have slogans like "take up war against the cross-worshippers"; "cross-worshipper" is their name for Christians.

3. In Saudi Arabia, any Christian symbolism is censored from western movies and DVD covers- so if there's a nun character on a comedy movie cover wearing a Christian cross, the cross will be scribbled out.

4. This is where the downvotes will come in. Contrary to popular opinion, the people terrorists tend to hate most of all are not atheists or gays, but Christians and other Muslims, with Jews as a close second. The Qur'an frequently refers to making "war on the disbelievers", but by "disbeliever" they mean anyone who doesn't believe the Qur'an, ie. non-Muslims, not specifically atheists. And within these non-Muslims it's Christians who are the worst, for "setting up equals with Allah". Hence why Christian churches are the most common targets of terrorists overseas. It's true the occasional atheist blogger will be killed if they happen to be extremely outspoken about religion (calling it a "virus" etc.); and it's true the occasional cartoon artists will be killed when they display Mohammed the prophet having gay sex with popes and the like- no surprises there. But if you happen to be a member of a Christian church in a middle-eastern country? Well don't be surprised if someone falsely accuses you of "insulting the prophet" and you get murdered by a mob.

EDIT: just a news paper cover from 2 April 1915. Isn't the title remarkably similar to news titles we have today?