President 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."
He has repeatedly leveled SOB-laced tirades against Obama, U.N. Secretary-General Ban Ki-moon, a few outspoken opposition politicians and human rights advocates, while heaping praises on Chinese and Russian leaders.
There were no expletives in his late Thursday speech, but Duterte still sounded mean toward critics.
When asked for his message for a Filipino beauty queen who won the recent Miss International pageant, Duterte said he was proud. "Many Filipinas are beautiful, but all of you there in the human rights commission are ugly."
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.