I think with criticising faith, it's a very delicate balance that some people don't get right.
Personally, I don't have a problem with challenging the accuracy of religious beliefs, and I don't have a problem with challenging the acceptability on behaviour of people that they justify by their beliefs.
I contrast that with sneering and prejudice.
One problem in terms of people of faith being criticised unkindly is that some very prominent people of faith, from an array of religions, have, over the centuries done precisely that in relation to people with other faiths or other versions of their faith.
When done simply as academic argument, it can be civilised and even help people of faith to clarify for themselves why they believe what they do. When not done that way, it can cause great hurt.
The examples I usually use in the context of a Christian culture are firstly, the amount of bloodshed and war even in the UK over the centuries, based around Catholic vs Protestant, mostly, in my opinion, using religion simply as a proxy for political identity. But that makes it no less real.
The second, rather less bloody, example I choose is unitarianism vs trinitarianism. Most trinitarians I know, if pushed to debate, struggle to see unitarians/Unitarians as Christians. But then that gets complicated when you get into the issue of what is or isn't unitarian anyway, and the more so when you get into what I'll call the physics and biochemistry of trinitarianism. Theologically aargh. Not in our country a war thing, but still sometimes a friction thing.
And in the midst of that are people who, just as some Christians feel in some way misunderstood, maltreated, disadvantaged etc. by different versions of Christianity, thus there are also, in this country, non-Christians, including atheists, humanitarians etc., who feel that way in relation to Christianity as a whole. That can trigger response in kind, which may also be re-directed towards others of faith that haven't maltreated them.
And what I say about Christianity applies within and between other religions.
As I say, I don't justify hostility, sneeriness, nastiness etc. in relation to different belief systems, I just say that I can see how easy it is for polite but heartfelt debate and expression of belief/non-belief to tip over.
That being said, I make no claim to know much about the specific expressed views of either Pratchett or Gervais.