Religion is one of the chiefest aspects of a culture, one that is quite deeply-rooted among many. To take away that religion is viewed by many as destructive of the culture in general, and probably the biggest way that people try to do this is to outlaw the religion, banning the worship of the religion's deity or deities and the practices of the religion in general.
The reasons for doing this vary:
One culture has taken over another, and in order to facilitate assimilation into the prevailing culture and stamp out rebellion, religion is among the many cultural aspects the other culture may outlaw, usually to force the populace into following the other culture's religion. Sometimes this works in reverse, with the ban on religion being a way to try to halt the encroachment of another culture on the other's way of life.
Another religion has taken hold in the land and sees the other religion as competition, often leading to the other religion being declared evil and to be wiped out.
Sometimes, in fantasy settings where Gods Need Prayer Badly, this may be an attempt to weaken or outright kill a god by denying him or her the worship he or she needs.
This usually results in the banned religion being driven underground, as people are naturally resistant to attempts to stop them from practicing their own religion and believing in or worshipping their own gods. As a result, this may be a source of tension between the people and the powers that be that if left unchecked (or aggravated by other factors) may very well lead to rebellion.
In cases involving a genuine, overt Religion of Evil, as opposed to a faith that is merely painted as such by those in power, this trope is usually not portrayed in the same negative light as banning a faith for other reasons. In such cases, the faith's priesthood will be given ample Kick the Dog moments, usually involving Human Sacrifice, to make it abundantly clear it really is deserving of prohibition.
Compare Outgrown Such Silly Superstitions. Also compare Ban on Magic. Not to be confused with Ban on Politics, which is a formal rule against discussion of sensitive topics such as politics and religion on internet forums and polite conversation due to the tendencies of such discussion to lead to fights.
When adding Real Life examples, please list historical examples only, and keep in mind the Rule of Cautious Editing Judgement due to the very sensitive nature of religion.
In V for Vendetta, England has transformed into a fascist dictatorship. Among the many things the government has made illegal, even possessing a Qur'an, let alone following the Islamic faith, is punishable by death.
During the reign of the Galactic Empire in the Star Wars universe, the Jedi were hunted down and driven to near-extinction by Imperial forces, their religion dwindling from universally recognized to often ridiculed as old superstition. Emperor Palpatine and his right hand Darth Vader were members of the evil Sith order, the ancient enemies of the Jedi.
Averted in Joust. The Tians allow Altans in captured territory to continue to worship their own deities, to avoid the problems that would result from underground worship. (The fact that the Tian and Altan pantheons are almost identical doesn't hurt, either.)
In David Eddings's Belgariad and Malloreon series, the Bear Cult, based on a misguided worship of the Alorn's god Belar, has to be periodically suppressed for its fanaticism.
In The Witcher universe, Coram Agh Tera, the Cult of the Lionhead Spider, is a forbidden religion in many of the civilized nations due to its practice of Human Sacrifice, and while the persecution is not as intense as it has been in the past, very few places will allow Coram Agh Tera cultists to preach openly. The government of Temeria is particularly keen to suppress the cult within their borders, and membership of the Lionhead Spider cult is a crime akin to murder.
Downplayed in the Star Carrier series, where the Terran Confederation's "White Covenant" law means that, while religion isn't banned outright, many of its common practices are. In particular proselytizing, many missionary activities, and conversion by threat or force are considered violations of basic human rights. This came about after Islamic terrorists nuked several major cities and set off World War III, and understandably doesn't sit well with a lot of religious groups (the Muslims especially, since it bans a core tenet of the faith, to bring the word of Allah to the infidel).
Mythology and Folklore
Defied by Dionysus in Greek mythology, who was known for killing rulers who made worship of him illegal.
The Church of the Silver Flame rules Thrane as a theocratic state, and while religions like the Sovereign Host are allowed within its borders, their adherents find life less comfortable, and few other religions, particularly the Blood of Vol whose association with the undead is utterly abhorrent to the Church, survive for long.
In the backstory of Warhammer 40,000, the God-Emperor of Mankind promoted state atheism, banning religion in an attempt to starve the Chaos Gods to death. It didn't work due to a research failure on his part: the Chaos gods are fueled not by prayer, but by emotion. The Imperium now ironically worships him, though other religions are still not permitted.
There are actually countless variations on the cult of the Emperor, as establishing a single version on the literally billions of planets is impossible. Instead, whether or or not a particular cult is heretical or not is basically up to the local governor / Ecclesiarchy, and the Inquisition intervenes if they think they're getting too lax. This has the unfortunate side effect of allowing genestealer cults to flourish, as unlike Chaos cults who are dead-set on toppling the Emperor and whose acts are rather obvious, the genestealers are allowed to keep worshipping their father figure, which then leads to the Tyranid fleets attacking.
Shadowrun: The elven nation of Tir Tairngire banned the Universal Brotherhood cult within its borders. They had good reason to: the organization in question is devoted to converting human beings into insect spirits.
In The Elder Scrolls V: Skyrim, the Empire of Tamriel has banned the worship of Talos, a god who was once a human (the guy who founded the Empire, in fact!) that ascended to the ranks of the other Eight Divines, in its lands, one of the terms of the White-Gold Concordat that the Empire was forced to sign after the Great War by the High Elven Aldmeri Dominion, who refuse to acknowledge Talos' ascension as the Ninth Divine and despise humans as well as pretty much anyone else who isn't an Altmer (or sufficiently Altmer for their liking). In most of the Empire, this isn't such a great deal, but in Skyrim, where Talos was originally born, it is the engine driving the civil war that rages across the land, as people are quite understandably pissed about the Thalmor, the agents of the Dominion, being allowed to go around and arrest people or worse for worshipping Talos, and resent the Empire for allowing this to happen. The Thalmor are attempting to unmake Talos by denying him his worship, with the ultimate goal of doing the same to humanity itself, even if it means destroying the world.
Anton's campaign in Might and Magic: Heroes VI begins with him being declared Duke of the Griffin Duchy and outlawing any religion except the worship of Elrath, the Dragon of Light. This upsets the orc tribes who live in the lands bordering Anton's Duchy, who prefer ancestor worship over organised religion, and Baron Djordje, who maintains temples to Elrath's brother Ylath, Dragon of the Skies, in his lands.
In the Command & Conquer: Tiberian Series, the Brotherhood of Nod (as much of a religious organization as a terroristic one) is considered illegal in all territories under GDI control or influence. After the Second Tiberium War, with the world's division into Zones, the GDI-controlled Blue Zones enforce their ban on Nod's teachings, while the Yellow Zones not under GDI occupation are effectively a Nod theocracy.
In Star Wars: The Old Republic, the Revanites are an illegal cult within the Empire that follows the teachings of Revan, a man who had been both Jedi and Sith almost three hundred years earlier.
Also on Voss, the dreamwalkers practice forbidden rituals.
In Futurama, The Church of Trek became an incredibly popular and powerful religion...so powerful it started taking over governments. Since The Trek Wars prior to the start of the series, any reference to The Church or its sacred texts are explicitly banned.
Christianity was banned in the Roman Empire until Emperor Constantine claimed to have been led to victory by the Abrahamic God after seeing Christian symbols in a vision. He legalized it, though he didn't convert himself until he was on his deathbed.
As Karl Marx famously called religion "the opiate of the masses", communist governments including the Soviet Union have typically outlawed religion in favor of devotion to the state. This didn't stop the people of those nations from practicing it, of course, but it did result in persecution of those that the government caught doing so.
Although Catholicism and Protestantism are both part of Christianity, for a while, some European countries would adhere to one and ban the other with extreme prejudice. Anticlericalism, a reaction against the traditional influence and position of the Catholic Church, has taken place in many countries in waves since the Protestant Reformation.
Nazi Germany was not too fond of anything besides Protestant Christianity, and even that was deemed to be in need of editing. In particular, they were notoriously anti-Semitic, and outlawing the religion of Judaism was only the beginning of worse things to come for them.
Japanese leader Toyotomi Hideyoshi banned Christianity in 1587, ordering all Christian missionaries to leave the nation. He saw the religion as a threat to his dream of unifying Japan, and politically, this was done to reduce the influence of the Christian daimyo of Kyushu. A decade later in 1597, Hideyoshi had twenty-six Christians, known today as the Twenty-Six Martyrs of Japan, crucified as an example to native Japanese seeking to convert to the religion.
Missouri Executive Order 44 of 1838 stated that all Mormons in Missouri "must be exterminated or driven from the state," essentially outlawing Mormonism in the state. It wasn't officially rescinded until 1976.
In the Tanakh (Old Testament to Christian readers), worship of anything other than the one and only God was illegal according to many laws and decrees by the prophets and is listed as the First Commandment ("Thou shalt have no other gods before me."). But for much of Israel's history many rulers not only allowed worship of other things but actively facilitated it. They were okay with magi even in periods where they otherwise had officially banned all other forms of worship too, since Cyrus The Great was viewed as God's servant foretold by Isaiah. Today Israel officially allows any religion to be practiced, but accounts from self identified "pagans" claim they would be cut to pieces if they publicly admitted to worshiping someone like Anat or Thor.
In Christendom anything but Christianity was discouraged and was often made illegal. Martin Luther was known for initially catering to Jews but then ordering their expulsion later on. The Spanish Empire notably expelled all non Christians after the Reconquista, the reconquering of all the areas settled by the Muslim invaders.
According to Sahih Muslim, Muhammad Ibn Abdullah had all Jews and Christians expelled from the Arabian peninsula and fought against the Arab idolaters until the only ones left alive gave up and converted. Whatever the case there are no public churches in Saudi Arabia to this day, because they are not allowed, and though most orthodox Muslim countries allow specific religious practitioners to live inside their boarders as dhimmi (non-Muslims), the list for eligible dhimmitude usually is not very long.
In around the 4th century CE the Sassanid Persian Empire was intolerant of any religion other than Zoroastrianism, enforcing a ban on them and attempting to force its Christian Armenian subjects to convert. The ensuing rebellion, led by Vartan Mamikonian, ended in a Pyrrhic Victory for the Persians and eventually led to Persia becoming more lenient due to their needing Armenia's cooperation in dealing with the invading Huns. Modern Armenians credit Vartan's revolt for saving Armenia's religious identity.
Five hats means that five tropers think it is ready to publish.
You are saying that you think this draft is ready to be published. That means the description is not ambiguous,
it doesn't duplicate an existing trope, there are at least three examples, and the title makes sense.
Is that what you meant to do?
You are saying this draft has a ready-to-publish hat it does not deserve and you are taking it back.