...What's up with that?
It's called taking-up sides against Your Own.
In all probability...
Even if you do not hold religious beliefs, you are a Westerner, and, like the rest of us, the inheritor of Western culture and tradition and morals and philosophy, which has a very strong connection to Christianity, as that belief system evolved and spread throughout Europe and its successor domains in the New World and Oceania.
When you equate Islam with Christianity - an erroneous position, quite arguably - you take sides against your own, in favor of those who would see us weakened and fall.
That does not go down very easily, amongst your fellow countrymen and fellow inheritors of our common Western culture.
Not exactly Rocket Science, when you get right down to it, is it?