A stereotype, but... well, religious.
The most obvious and prominent example is the Corrupt Church, which involves portraying all clergymen, especially Christian ones, as sanctimonious fanatics and/or hypocrites. When Religious Stereotype is in full effect in a work of fiction, expect all Protestants to be poorly-educated racist rednecks or The Klan - don't bring that civil rights movement crap here! All Catholic priests will be pedophiles, which will make one wonder how the Church functions if all its priests are, safe to say, busy. At worst, "Catholic" inevitably means The Spanish Inquisition and Knights Templar screaming "Burn The Heretic!"
And it doesn't get any better when you leave Christianity, oh no! As we all know, If you're a Muslim, then you're a terrorist - or if female, completely wrapped in a burqa because any form of skin exposure is evil. Jews will probably be... well, many times Jews will be found where you least expect it in your characters, but to be a real stereotype, expect them to be nerds obsessed with saving money. More generally, though, Jews will get assigned the more positive (or at least funnier) stereotypes these days rather than the awful negative stereotypes of the past.
Well, your fictional character says to himself, to hell with this, I'll just be an atheist!... Nope, buddy, you're still stuck. Atheists are evil in most portrayals of fiction, as Straw Nihilists with no sense of morality, obsessed with Evolution, hate religious people (and may even want to purge religion ala Stalin) and will always have a sad backstory for being an atheist.
Still, note that all stereotypes are not bad. Just as common a stereotype is that of the Buddhist monk who literally knows everything worth knowing and helps the Hero, while another common one is the Hindu guru who is "in tune with the universe".
This trope almost always results in Unfortunate Implications of the religious variety (especially if the author uses these stereotypes to boost up their own favorite side), to the point that its inclusion in a work of fiction should probably set you on high alert. Buyer (and writer) Beware!
If a negative stereotype was created just to make the hero look good, or the Creator's views seem right, then it is a Strawman. See also Christianity Is Catholic, and any of the Useful Notes series on the religions for what the religion is actually like. See also evil religions, good religions, HeelFaith Turn... ah, just go check out the Religion Tropes. Religious Stereotype is a part and parcel of all of them.
- A particularly hated storyline in X-Men had an anti-mutant group planning to have Nightcrawler elected as Pope (!) and then cause a false Rapture to ruin both The Church and mutantkind. A horrible research flub, since the Roman Catholic Church doesn't believe in the Rapture, and it certainly doesn't have any end of days prophesies about the Pope being a demon.
- Similarly, an earlier X-Men graphic novel (God Loves, Man Kills) also had a fundamentalist persecute mutants. Ironic in that the villain of the story was later used in the second X-Men movie, but as a military leader. It's not quite as extreme an example as most, as the bigoted fanaticism of Stryker was contrasted with the benevolent devoutness of Nightcrawler and others, and the story ends with his own followers turning against him when he crosses the Moral Event Horizon.
- Garth Ennis indulges in this from time to time, most notably Preacher.
- Carrie's mother, Margaret White, in the movie but more pronouncedly in the book, is a psychotic religious nut who believes periods are punishments from God. It's not clear if she's a Fundamentalist with some very whacked-out views or a member of a vaguely Christian cult. Regardless, it's not the most flattering portrayal.
- Mrs. Carmody from The Mist screams this. Not hard to tell what Stephen King thinks of certain religious types.
- The characters of Ruth and her father in Paul.
- The president in Escape from L.A..
- This trope is the reason behind the Outgrown Such Silly Superstitions trope in science fiction and fantasy: the assumption that smart people can't be religious. Belief Makes You Stupid is part and parcel of the same thing.
- Stephen King has indulged in this in just about every novel he's ever written; you can pretty much always expect at least one nasty (at best) or psychotic (at worst) fundamentalist Protestant character, who often doubles as the "overbearing parent", without any more positive examples to provide a contrast. To be fair, though, he averts this quite heavily in Desperation, The Stand, and Cell. It does have that crazy bible lady at one point, but Alice averts this. King himself has said he believes in God. Also of note is Father Callahan, who becomes one of the most sympathetic and, ultimately, heroic characters of The Dark Tower.
- The Regulators, an alternate-universe companion to Desperation, does feature Reverend Hobart, a fundamentalist neighbor. When his son Hugh steals Seth's toy, there is evidence that Hugh was beaten for "giving in to the tempting voice of Satan." Reverend Hobart is then willing to let Seth's aunt Audrey spank Hugh again. But when Audrey merely wants him to say "I was wrong, I'm sorry, and I won't do it again," Reverend Hobart protests that this is browbeating.
- Muslims in Caliphate are all depicted as cruel, violent, misogynistic, perverted, and religiously intolerant. The only exceptions are Besma (who is genuinely heroic) and her father Abdul (who is semi-decent).
- His Dark Materials portray religion as a Corrupt Church trying to control everything through conspiracy and believing everything they can't understand to be heresy that must be eradicated. No character affiliated to religion in any way (down to a random priest in a village) is presented under a good light.
- A Wolf In The Soul presents barely-religious Jews as leading somewhat meaningless and hypocritical existences.
- Similar to the Caliphate example above, in In The Year 2050: America's Religious Civil War Muslims apparently live only to oppress and murder non-Muslims, as even after they secure Congress, the Presidency, and a majority of the population of the US, there are still weekly suicide bombings President Osama bin Muhammed needs to call off to trick the infidels into believing Islam is a peaceful, friendly religion, while imposing brutal Sharia law.
- Played with on an episode of Bones where an intern to the Jeffersonian is discovered to be faking a thick accent to avoid annoying questions about being both a devout Muslim and a highly educated man.
- Played with on an episode of Veronica Mars where Veronica's investigation involves the conservative preacher father of a friend. The preacher's assistant fits the fundie stereotype to a T as well as being the villain of the episode. On the other hand the friend's father turns out to be both honest and compassionate.
- On True Blood the Fellowship of the Sun seems to be a compilation of every possible negative stereotype the authors could find for southerners, religion, and/or southern churches.
- On The Big Bang Theory, Sheldon's mother embodies every stereotype of a born-again evangelical Christian. And redneck.
- Aunt Esther from Sanford and Son is portrayed as a judgmental, Bible-thumping loudmouth who usually wears a very unpleasant lemon-lips facial expression.
- Averted in Everyday Heroes with Carrie Pelosi who, although quite devout, is the type of Christian who believes in teaching by example rather than preaching.
- Seymour of Sinfest is definitely a caricature of the stereotypical evangelical Christan. Lil' Evil could also count, as your average evil Satanist stereotype. Characterization Marches On for both, though that likely because of the comic's shift away from religious humor.
- Gil Marverde from Ava's Demon has several stereotypes regarding his worship of TITAN. On the negative side he is nigh-fanatic, misguided, a bit out-of-touch with reality and doesn't take well to people pointing out the logical flaws of his religion. On the positive side, he's the genuinely nicest and most selfless of the main characters as well as an aspiring doctor.
- The majority of Christians on Family Guy are depicted as conservative, hypocritical, and intolerant. Jews get much better treatment, but the most recurring Jewish character is the whiny, hypochondriac, money-obssessed Mort Goldman. And all Muslim characters are Middle Eastern terrorists.
- Ned Flanders in The Simpsons became this over time, going from a nice guy who happened to be Christian, to being a living stereotype as Writer Onboard increasingly asserted its presence, naming a trope in the process. And all Buddhists will be the Positive stereotype where their all super enlightened peaceful people with no desires and no trouble getting on with anyone ever. Lisa, the most relevant Buddhist character, is not always presented in a good light either, and Ned himself is hardly presented as a bad person (a little bit of a religious nut, but not mean or evil) and it's very popular among Christians themselves in a probably case of Misaimed Fandom.