Years ago I had a high-school English teacher--Thomas Chafee by name--who
began every class with a short quote scrawled on the blackboard. We would
then spend a few minutes discussing it. Thirty-eight (!) years later I
still remember a few:
"There is none so holy as a reformed whore"--Unknown
"Life is nasty, brutish and short"--John Donne
AND
"A fanatic is one who redoubles his efforts after losing sight of his
goal."--MT

I suspect this is the translated quote, though I am not near a Bartlett's
et al to reference it against the key word "fanatic"

MacKinnon Simpson