Duterte, who famously called both Pope Francis and US President Barack Obama "sons of whores," said that God had spoken to him while he was traveling on a plane.
The Philippine president said his Damascene moment came as he traveled home to the Philippines on a plane from Japan.
"While I was coming over here, everybody was asleep, snoring," said Duterte. "A voice said that you know... if you don't stop epithets, I will bring this plane down now. And I said who is this? Of course, it's God."
"Okay, so I promised God not to express slang, cuss words. So you guys hear me right always, because a promise to God is a promise to the Filipino people."
Tough promise to deliver
When Duterte's vow was met with applause, he warned: "Don't clap too much or else this may get derailed."
Indeed, it would appear to be a tall order for the 71-year-old, who made a similar pledge in June, when his victory in the presidential election a month earlier was made official.
He had said at the time that he had been enjoying his last moments as a "rude person."
"When I become president, when I take my oath of office ... that will be a different story," he said. "There will be a metamorphosis."
Since then, Duterte has launched foul-mouthed tirades at figures including UN Secretary-General Ban Ki-moon and the US ambassador to the Philippines. Meanwhile, he has sought to build alliances with China as well as Russia, saying he wants the Philippines to break away from its long-standing alliance with the US.
rc/kl (AP, EFE, epd)