Muhammad Ali sits in his Chicago home, reading "Message to the Black Man," by Elijah Muhammad, leader of the Black Muslim sect, in an undated photo. Getty Images

The faith of Muhammad Ali has been among the most contentious elements of the famed boxer’s life. Perhaps the most prominent public figure in modern times to convert from Christianity to Islam, Ali caused a stir when he changed his name from Cassius Marcellus Clay Jr. in 1964, but he has had an evolving relationship with the Muslim faith.

"They call it the Black Muslims," said a 22-year-old Clay at the time. "This is a press word. It is not a legitimate name. But Islam is a religion, and there are 750 million people all over the world who believe in it, and I am one of them."

After defeating Sonny Liston by a technical knockout in a controversial heavyweight title fight, Ali would join the Nation of Islam, whose doctrines of racial separation deviate from orthodox Islam. But he would later convert to mainstream Sunni Islam in 1975, and then to the Sufi sect in 2005.

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Muhammad Ali was a member of the Nation of Islam in the 1960s and forged an intense friendship with Malcolm X. Getty

In the recently released book “Blood Brothers,” historians Randy Roberts and Johnny Smith chronicled the intense friendship of Muhammad Ali and social activist Malcolm X, who had “magnetized Clay, drawing him toward the inner circle of the Nation.”

In the early 1960s, there was a fear from Ali’s camp that his relationship with the Nation of Islam would jeopardize his chances of a shot at the heavyweight title. Not only did Ali fight and defeat Liston, he would later emerge as a political figure in 1967 for refusing induction into the armed forces during the Vietnam War, and leading to a three-year suspension.

"The relationship between Cassius Clay and Malcolm X signaled a new direction in American culture, one shaped by the forces of sports and entertainment, race and politics," Roberts and Smith wrote. "Under Malcolm's tutelage, [Ali] embraced the world stage, emerging as an international symbol of black pride and black independence. Without Malcolm, Muhammad Ali would have never become the 'king of the world.'"

But when Malcolm X broke away from the Nation of Islam, the friendship with Ali also broke off. After Malcolm X was assassinated in 1965, Ali expressed regret that he never had the chance to mend the friendship, according to Roberts and Smith.

In a Facebook posting, Yasir Qadhi, a professor of religious studies at Rhodes College in Memphis, Tennessee, fondly detailed Ali’s positive influence on American Muslims.

“If the only good that he brought was to bring a positive image of Islam, and to spread the name of our beloved Prophet (SAW) in every household and on every tongue in the world, it is a life that is indeed enviable,” Qadhi wrote.

He praised Ali’s “positive political activism” and “preaching truth to power.”

“He converted to what he thought was Islam at a time when Islam was an unknown religion; then he became Sunni after Malcolm X introduced him to mainstream Islam, and he's been a proud and public Muslim ever since.”

As recently as December, Ali maintained his role as a proponent for Muslim rights. He addressed Republican presidential candidate Donald Trump’s controversial stance on halting immigration of Muslims with a statement that had the headline: "Presidential Candidates Proposing to Ban Muslim Immigration to the United States."

Ali did not mention Trump by name, but it was clear that his intention was to promote a positive image of Muslims and denounce radical Islam and ties to terrorism.

“I am a Muslim and there is nothing Islamic about killing innocent people in Paris, San Bernardino, or anywhere else in the world. True Muslims know that the ruthless violence of so called Islamic Jihadists goes against the very tenets of our religion.

“We as Muslims have to stand up to those who use Islam to advance their own personal agenda. They have alienated many from learning about Islam. True Muslims know or should know that it goes against our religion to try and force Islam on anybody.

“Speaking as someone who has never been accused of political correctness, I believe that our political leaders should use their position to bring understanding about the religion of Islam and clarify that these misguided murderers have perverted people's views on what Islam really is.”