The unclassified comments by Khalid Sheik Mohammed, who has spent the last decade in U.S. custody in the military prison facility in Guantanamo Bay, Cuba, were made about 18 months into the pre-trial phase of his military commission trial, the Los Angeles Times reported Tuesday.
Pentagon officials said Mohammed wrote the document in October and asked that copies be delivered to the court officials.
The Koran, Mohammed wrote, "forbids us to use force as a means of converting" others.
"[Truth] and reality never comes by muscles and force but by using the mind and wisdom," he wrote.
His writings, the Times said, run counter Mohammed's calling for young Muslims to embrace violence. He also has bragged he was behind the 2001 attacks and personally beheaded Wall Street Journal reporter Daniel Pearl.
Mohammed also lashed out at same-sex marriage, abortion and what he sees as the moral disintegration of the West, the Times said.
Mohammed said the document was the first of three writings. Future writings will defend the Sept. 11 attacks and dispute the validity of the U.S.-led invasions of Iraq and Afghanistan.
He said his time at Guantanamo Bay has not been wasted and he was "neither sad nor distressed."
"I have been with the Only One True God," he said. "I am very happy in my cell because my spirit is free even while my body is being held captive."