Green’s Dictionary of Slang

(Aus.) a Baptist.

1953		Baker Aus. Speaks.
1972		A. Chipper Aussie Swearers Guide 61: The first thing any full-blooded Aussie does when handling an amen snorter is shorten his denomination to size and add an ‘O’. As in Bappo, Congo, Metho, Presbo, Protto, Salvo, etc.