Duterte revealed that he made the pledge to control his bad language after hearing a voice speak to him on a flight from Japan to his home town of Davao.
The president said the voice told him: "'if you don't stop [swearing] I will bring this plane down now".
When he asked who the voice belonged to, Duterte claims the response was, "It's God".
He explained: "So, I promise God to...not express slang, swear words and everything. So you guys hear me right always because (a) promise to God is a promise to the Filipino people."
Here are some of the most shocking slurs President Rodrigo Duterte has said during his political career:
On Hitler: "Hitler massacred three million Jews. Now, there is three million drug addicts. I'd be happy to slaughter them".
On Pope Francis: After being stuck in traffic in Manila for five hours during the Catholic pontiff's visit to the Philippines, Duterte said: "I wanted to tell him to go home and not visit us again." Before branding the Pope a derogatory insult.
On Barack Obama: After the US president criticised Duterte's war on drugs, the Philippine president called him a "Son of a b----".
Duterte made a similar pledge in June after winning the May 9 presidential elections overwhelmingly on a pledge to deal harshly with drug dealers.
At the time, he said he was enjoying his last moments as a "rude person" because "when I become president, when I take my oath of office ... that will be a different story. There will be a metamorphosis". It did not take long for Duterte to break the promise.
After his latest pledge was applauded by those at the press conference, Duterte warned: "Don't clap too much or else this may get derailed."