Green’s Dictionary of Slang

(US) a derog. term for an overly religious person, esp. a proselytizing teetotaller; occas. attrib.

1922		R. McAlmon ‘Backslider’ in A Hasty Bunch 4: When he wasn’t drunk entirely he managed to be a bit ironical about the ‘Christ-shouter at the barn’.
1927		(con. 1900s) S. Lewis Elmer Gantry 26: Heh! I suppose you’re a Christer too!
1934		H. Miller Tropic of Cancer (1963) 64: It doesn’t matter to me whether he’s a chauvinist, a little Christer, or a near-sighted pedant.
1949		J.H. Burns Lucifer with a Book 79: The strong old executive and the querulous Christer.
1951		Cornell (University) Daily Sun 23 Oct. 4: Non-drinkers are called all sorts of names – one in popular usage is ‘Christer’ [W&F].
1956		H. Whittington Forgive Me, Killer (2000) 69: I’m not a Christer [...] I’m no better than any other man.
1960		(con. 1940s) G. Morrill Dark Sea Running 96: The Old Man was one of them holy-christers.
1967		M. Braly On the Yard (2002) 83: Make it to church. Some of them Christer broads are all right.
1979		G. Wolff Duke of Deception (1990) 206: ‘We’ kissed off the pushers, strivers, tweedbags [...] Christers, doers.
1986		S. King It (1987) 190: The Neibolt Street Church School, which was run by people Ben’s mother called ‘the Christers’.
2001		J. Stahl Plainclothes Naked (2002) 124: Lemme tell you, that Christer looks like he crawled out of a cement mixer.
2007		C. Goffard Snitch Jacket 153: A bunch of Christers protesting.