Green’s Dictionary of Slang

naïve, gullible, i.e. typical of the evangelical church and its devotees.

1999		N. Cohn Yes We Have No 340: It’s not [...] happy-clappy mindlessness.
2000		Indep. on Sun. Real Life 6 Feb. 4: Our happy-clappy, talk-show-riddled society.
2004		R.G. Barrett Rosa Marie’s Baby (2013) [ebook] It was all soul music and seemed to be a happy-clapping, praise-the-Lord station.
2023		N. Cohen Writing from London 2 Dec. 🌐 [P]retending that Jews and Palestinians will go on to live together in some happy-clappy, multi-ethnic and multi-confessional state.