The foul-mouthed Philippine president, who once called the pope a "son of a bitch” and told Barack Obama to "go to hell,” says he has promised to God he won’t spew expletives again.
Mr Duterte’s zest for outrageous declarations has played well at home but left many outsiders, notably Manila’s traditional allies in Washington, confused about his real thoughts and aims. His tough talk also has domestic consequences: his bloody war on drugs has claimed thousands of lives, including a town mayor and nine of his associates on Friday.
Duterte made the stunning pledge on arrival in his southern hometown of Davao city late Thursday from a trip to Japan.
While flying home, he said he was looking at the sky while everyone was sound asleep, some snoring, and he heard a voice that said "‘if you don’t stop epithets, I will bring this plane down now.”
"And I said, ‘Who is this?’ So, of course, ‘it’s God,'” he said.
"So, I promise God to … not express slang, cuss words and everything. So you guys hear me right always because (a) promise to God is a promise to the Filipino people.”
Duterte’s vow was met with applause, but he cautioned: "Don’t clap too much or else this may get derailed.”
Mr Duterte, who took office at the end of June, has called both Pope Francis and Mr Obama "son of a bitch” in separate outbursts. He also had to apologise for likening himself to Adolf Hitler and saying he would be prepared to kill millions of drug addicts.
Mr Duterte’s homecomings to Davao have taken on something of the status of confessionals, in which he attempts to clarify or moderate statements he made during his travels. On his return from China last week, he said he had not really meant it when he dramatically announced the Philippines’ "separation” from the US, its security treaty ally.
He shocked Roman Catholics in 2015 when he fired an expletive while expressing his disgust over a traffic jam that trapped him while Pope Francis was visiting Manila.
"I wanted to call, 'Pope, you son of a bitch, go home. Don't visit here any more'," he told supporters.
He later apologised after Filipino bishops expressed shock and outrage.
It is not certain if the 71-year-old president, who has been compared to US Republican presidential candidate Donald Trump because of his brash language, can keep his promise.
Duterte made a similar pledge in June when it became evident he had won the May 9 presidential elections on a pledge to end crimes, especially illegal drugs and corruption.
It did not take long for Duterte to break the vow.
Asked if the days of his cursing the U.S. and the E.U are over, Duterte didn’t answer clearly.
"I do not want anybody reading my mind because I couldn’t make a smart move anymore. But it’s all calibrated, it’s all about timing,” Duterte said.