"When I was about nine years old, I would wake up in the middle of the night and go outside to wait for an angel or a revelation from God," he wrote in the book. "I would sit on the front porch, look up at the stars and wait for a message. I never heard anything, but I never lost faith, because the feeling was so strong in my heart."
Born Cassius Clay Jr., the boxer's spiritual journey would lead him to become a Muslim in 1964. He became an outspoken member of the controversial Nation of Islam, an African American religious movement that fuses together aspects of traditional teachings that mainstream Muslims follow with the black pride movement. Elijah Muhammad, a Nation of Islam leader, gave Ali the name he would use for the rest of his career.
Religion continued to play an important role in the sporting great's life. In 1967, Ali pitted his fighting spirit against the U.S. government by refusing to to serve in the military during the Vietnam War, citing his religious beliefs. He was banned from boxing for three years for this bold stance -- but used that time to speak out against the war on college campuses.
In his later years, Ali served as a diplomat, philanthropist and an advocate for interfaith understanding. Most recently, he publicly criticized presumptive GOP presidential nominee Donald Trump about his proposal to ban Muslims from entering the United States. Ali called on political leaders to "bring understanding about the religion of Islam.”
His death was announced Friday night. He was 74. In honor of his life and legacy, here are 12 powerful quotes about faith from this extraordinary man.