Why I have frequently criticized Christianity but not Islam in my life:
(reposted in group by request of GD host)
It is pretty simple actually: Because I grew up with the Christian beliefs. I was born a Catholic, later became a Protestant, later became an Agnostic.
When I criticize or even mock certain parts of the Christian beliefs, it is because I know what I am talking about. I know the religion and its institutions, and the beliefs are beliefs which I was born into and later rejected. They are beliefs I am deeply familiar with, because I used to hold them myself. I was a kid who got pissed off from seeing "Life of Brian". I feel like I own those beliefs as much as any believer. They are mine to criticize or make fun of.
With Islam (or Hinduism, Judaism, Buddhism, Scientology, Ralienism...) it is different. Thinking about those will always be an intellectual exercise to me, but nothing more, since I did not grow up within their institutions. I may have a theoretical discussion about them, and criticize them, but to outright mock them will always feel like shitting on someone else's carpet to me.
= new reply since forum marked as read
The mocking mentions of some heresies are the only remaining records of those groups. The mockery is all that survived Theodosius's bonfires.
But to the point. Mockery educates, and educates painlessly. That's how it got under William Blake's skin.
"Mock on, mock on, Voltaire, Rousseau,
Mock on, mock on, tis all in vain.
You throw the sand against the wind
And the wind throws it back again."
He knew it wasn't in vain. 
