For the very same reasons that you should not mock "nationalities, race, gender, sexuality....."
Mockery: insulting or contemptuous action or speech
I'd almost bet your mother taught you it wasn't nice.
I'd also bet that most of us, from time to time, forget what we were taught.
Mockery is not necessarily the right word, but rather 'strongly questioned'. However my mother put me in Sunday school, and if you can choke down the contradictory and nonsensical song and dance routine, it gets real hard to take moral lessons from an organization with a predilection for pedophilia. I quickly realized what a mockery the whole thing was, and that I had more morals in one toe than the entire organization put together. Fast forward to the christian right simultaneously banging on about pro life and traditional marriage, and falling over themselves to worship serial adulterer and sexual predator Trump who promises to bomb the sh!t out of everything and kill terrorists' families, again making a total mockery of their own tenets. So maybe mockery is the right word, and exactly what they deserve, and they have no one to blame but themselves.
Nationality, race, gender and sexuality are not choices, religion is.
Ding ding ding!