They've been with us for a long time for sure. For example, the ubiquitous early modern conspiracy theory that was the commonly held idea in 17th century England that Catholics were, at any minute, ready to take over the country and supplant the laws and liberties of Protestants, converting them by force or massacring them wholesale. This particular theory was inspired by real events like the St Bartholomew's Day massacre in 1572 in France, the Spanish armada, the Gunpowder plot, as well as imagined 'preparations' like Charles I's alleged building up of an Irish army to suppress parlimentary opposition in the early 1640s.

Concerns over this ever present threat show up frequently in the political discourse of the era, but the culmination of it is really the "Popish Plot" of 1678. The entire conspiracy was fabricated by Titus Oates, who alleged that a group of Jesuits were planning to murder the restored King Charles II and replace him with his brother James, (later James II) a known Catholic, and in fact the last Catholic English monarch. Charles himself was mostly unconcerned and thought the idea absurd, remarking to his brother that they would not kill the popular king to place James on the throne.

Oates was later discredited and disgraced, but despite Charles' confidence that Oates was a liar, he was able to convince a court, and of course hysteria among much of the population took root. A faction in parliament who'd later become the Whigs, the first political party, attempted to exclude James from royal succession. This and the general anti-Catholicism was used to penalise and create civil and legal disabilities for Catholics that would persist for over a century, making this a very significant episode and wider concept in British and Irish history.