Discworld equivalent of the evangelical pamphlet Plain Truth or The Watchtower - the magazine with which the Jehovah's Witnesses bombard unsuspecting door-openers. Distributed around the city with outstanding zeal by Constable Visit-the-Infidel-with-Explanatory-Pamphlets and his co-religionist Smite-the-Unbeliever-with-Cunning-Arguments.
The Plain Truth was the magazine of the Worldwide Church of God, a breakaway Christian sect which, under its charismatic and authoritarian leader Herbert W. Armstrong (1892-1986), preached a form of evangelical Christianity distinguished by noticeably right-wing politics and a revival of the old British-Israelite cultural myth. (A major component of Armstrong's theology was British Israelism—the view that the white peoples of America and Britain are the descendants of ancient Israel, and are therefore the chosen of God, with full right to rule the world in His name) Unlike the Jehovah's Witnesses, the WCOG did not do door-to-door evangelism, preferring to distribute its magazine for free via news-stands and news agencies worldwide, and via advertisements in sympathetic conservative and right-wing publications such as the Reader's Digest. Anyone responding positively to the message was then nurtured by post and personal invitations to Church rallies. Effectively, this was doorstep religion via the Post Office (I wonder if Moist von Lipwig has entered into any lucrative distribution agreements with the Omnian church yet?)
Following the death of the Founder in 1986, the Church schismed. It has since, in the main, abandoned the world-view that caused it to be seen as heretical and heterodox by mainstream evangelical churches. It also dropped the somewhat anti-Semitic and racist streak that had been there under Armstrong's tutelage, possibly for the same reason as the Mormons - it was threatened with loss of tax privileges by an IRS which is otherwise indulgent to American religion - and has since become just another part of the generalised Religious Right in the USA. The Plain Truth is no longer published in its old form (the ideas were so wacky and off the wall as to make free and entertaining reading). Today, it's just another mainstream evangelical publication of the sort that manages to conflate belief in Jesus Christ with unwavering support for the Republican Party - ie, there is no God but Jesus and His earthly prophet is George W. Bush. or, these days when Bush is thought of as having been dangerously liberal, in Donald Trump.
Although several schismatic rump churches remain, holding the original flame of truth and unswerving dedication to the Prophet Herbert and the One True Path whilst decrying the heretics and schismatics who managed to trouser most of the money and assets, assisted by Satanic lawyers. So it goes...