“a hundred days of sermonizing, one fart” → a metaphor for how something that one has put great effort into can come to nothing after even a slight mistake, much as a hundred days of serious spiritual study can be lost to the winds

entirely, quite; completely; just like, so to speak, as though, as if

expounding the teachings of Buddhism / preaching

to expound the teachings of Buddhism / to explain to others one's opinion

one thing

flatulence, fart

(mildly vulgar) flatulence, fart

he / distal demonstrative, something far off removed from both speaker and listener: that / boyfriend

distal demonstrative, something far off removed from both speaker and listener: that, yon / particularly, male personal third person pronoun: he / by extension from he: boyfriend

he; boyfriend