First, it's art. The artist obviously had a bone to pick.

Second, as G59 pointed out, you are in a sub-reddit about atheism.

Third, religious doctrines, at least in the western world, tend to also carry with them other tacked-on ideologies, especially political ones (and when I say that, I mean compare the "right" and "left" in even my country [Canada] and note that our right wing seems leftist, and likewise your left wing seems extremely far right. And we are right next door - I do not think it's a coincidence that our most religious province, is less religious, than the least religious state.)

Also consider that christianity in particular, because of the large amount of money held by the religious right, has a very large influence on all other ideologies simply by the factor of wealth. (and often wealth=influence) All religions have this potential - and yes, most ideologies, but what ideologies are really prepared to compete with the big religions, especially christianity, for space in children's minds?

I'd bet those other ideologies would rather share the memeplex; especially because, one meme can spread the other, making both more virulent. And I assure you, christianity, is a virulent meme.