Excuse me? What did you mean 'just christian'? Even if he is Christian he shouldn't act like this, and Muslims don't despite Christians, they have many beliefs that are the same, the only difference is who they pray for and some minor things.
Yes it's a religious thing, and it's serious, if the author cared about their job and the thoughts of their readers, they wouldn't have posted this things, meaning they don't respect anyone.
It's not a false information, you can go and look at the pages of the authors and find it's true, you are the that one, speaking before you know what is right from what is wrong.
Just found out this manhwa’s author is an islamaphobe