I've tried to explain it before, but liberals have a certain narrative that they follow. They only care about hatred towards others if it fits that narrative. People they see as victims are "protected" and will trigger outrage if they are insulted. Other people who don't fit their narrative, get treated as second class citizens. Christians happen to be part of that group.
I hate to make this about race, but it is. Most liberals will say that to defend themselves, but you wont EVER see them attacking a region tat is seen as non-white. Christians, Catholics and Mormons get it so bad because they are perceived as being white.
Look at how Romney was attacked by the media for his religion. Has anyone ever seen that as acceptable? Of course not, but since it didn't fit the narrative, it was free game.