Religion and oaths
From God's wounds to fuck
"Swearing" originally meant taking an oath. The history of English bad language is the history of which words counted as swearing, and for most of it, fuck was not one of them.
The Middle Ages: oaths were the taboo
Medieval English speakers said fuck, shit, arse, piss and the rest without much embarrassment. Chaucer's Canterbury Tales (c. 1390) use swive for sex and queynte for the female genitals in plain narrative, and medical and legal texts of the period name body parts and functions without euphemism. Street names recorded the same words. What frightened people was swearing by God: "by God's wounds", "by His blood", "by Christ's nails". Theologians taught that a vain oath tore at the body of Christ afresh, and the Third Commandment made it a sin. Melissa Mohr's Holy Sh*t (2013) calls this the "holy" phase of English swearing, in which the worst thing you could say was religious and the "shit" words were merely rude.
The Reformation and after: the switch
From the 16th century the two categories traded places. Protestantism, then secularism, drained the force from religious oaths, which softened into zounds, gadzooks, gosh and golly. At the same time rising standards of privacy and politeness, the spread of print, and the growth of a middle class anxious about respectability made the body unmentionable. By the 18th century fuck was disappearing from dictionaries; by the Victorian period words for sex and excretion had become the strongest in the language, and "swearing" had come to mean them. The shift was complete when a 1960 jury could be asked, in all seriousness, whether the word fuck was fit for wives and servants.
Blasphemy law
England had a common-law offence of blasphemous libel until 2008. The last successful prosecution was in 1977, when Mary Whitehouse brought a private case against Gay News over a poem; the last attempt, against the BBC over Jerry Springer: The Opera, failed in 2007. The Criminal Justice and Immigration Act 2008 abolished the offence. Scotland abolished its equivalent in 2021, and Ireland removed blasphemy from its constitution by referendum in 2018. The word fuck was never the subject of blasphemy law; it is included here because the law's disappearance is the end of the "holy" era, and because obscenity law, which did concern the word, took over exactly the cultural space that blasphemy law vacated.
What religious speakers do now
Practising Christians, Muslims and Jews report, in survey after survey, swearing less than the population average and rating sexual swear words as more offensive. Religious communities are also the main source of complaints to Ofcom about language. At the same time the religious oaths have become so weak that bloody, which may or may not derive from "by our Lady", is now rated mild, and damn and hell are printable in children's books. In Quebec, uniquely, the church words remained the strongest: tabarnak (tabernacle), câlisse (chalice) and hostie (host) are the Québécois equivalents of fuck, and the English word is a mild loan beside them.
The third phase
Mohr's argument, and most linguists' now, is that the shift is happening again. The sexual and excretory words are losing force through sheer frequency, while words that attack people by race, disability, sexuality or gender have become the ones that end careers and reach courts. On Ofcom's scale the racial slurs and fuck sit in the same "strongest" band, but the public treats them very differently: one is heard on every football terrace, the other is not. Whatever the word for "swearing" means in fifty years, it probably will not centre on this word.