A dictionary of Proverbs (Oxford Paperback Reference)

ONCE a priest, always a priest
An act enabling the clergy of the Church of England to unfrock themselves (the Clerical Disabilities Act) was passed on 9 Aug. 1870.

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  1859 G. A. SALA Twice round Clock 290 The great case of Horne Tooke versus the House of Commons—’Once a priest forever a priest’. 1865 L. STEPHEN Life & Letters (1906) ix. As in this.. country we stick to the maxim, ‘once a parson, always a parson,’ I could not.. go in for law. 1916 JOYCE Portrait of Artist (1967) iv. You must be quite sure, Stephen, that you have a vocation because it would be terrible if you found afterwards that you had none. Once a priest always a priest, remember. 2002 Washington Times 14 June A1 ‘Once a priest, always a priest’ is a fundamental belief among Catholics. But that tenet may be tested as bishops consider firing hundreds of priests. ■ constancy and inconstancy
  

ONCE a whore, always a whore

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  1613 H. PARROT Laquei Ridiculosi II. cxxi. Well you may change your name, But once a Whore, you shall be still the same. 1659 N. R. Proverbs 82 Once a whore and alwaies a whore. 1754 World 31 Jan. 344 Supposing him to have acquired so much wealth, the proverb of ‘Once a whore, and always a whore’, is less significant. 1824 H. MARSHALL Hist. Kentucky II. v. ‘Once a prostitute, and always a prostitute,’ is a fair mode of argument—at least, among politicians. 1981 N. LOFTS Old Priory v. iii. There is a saying, ‘Once a whore, always a whore.’ -constancy and incontancy; wrong-doers
  

ONCE bitten, twice shy
The variant once burned, twice shy is particularly common in the US, perhaps influenced by a BURNT child dreads the fire.

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  1853 SURTEES Sponge’s Sporting Tour xxxvii. Jawleyford had been bit once, and he was not going to give Mr. Sponge a second chance. 1894 G. F. NORTHALL Folk­Phrases 20 Once bitten, twice shy. 1920 CONRAD Rescue III. ix. Once bit twice shy. He had no mind to be kidnapped. 1949 ‘S. STERLING’ Dead Sure xv. She was especially on her guard.. because she’d been victimized in a stupid swindle herself, recently. Once burned, twice shy, you know. 1981 H. ENGEL Ransom Game xvi. I can’t imagine why this man would be harassing my wife again, Mr. Cooperman. You’d think ‘Once burned, twice shy’, wouldn’t you. 1998 Times 7 Jan. 27/6 Once bitten, twice shy when thinking of certain shops. -experience
  

once see also you can only DIE once; FOOL me once, shame on you.. ; MEASURE seven times, cut once; MEASURE twice, cut once; THINK twice, cut once.