Muhammad Ali called himself the greatest long before he truly achieved greatness. Today, his claim rings true retrospectively and describes the boxer in his simplest and purest form as he is remembered on the sixth anniversary of his death on Friday.

"Muhammad Ali made boxing a worldwide sport," said Hall of Fame boxing trainer Freddie Roach. "He drew the fans in and brought more visibility to the sport and to humanitarian causes."

Born Cassius Marcellus Clay, Jr. on Jan. 17, 1942, in Louisville, Kentucky, Ali started boxing when he was just 12 years old. He racked up Golden Gloves boxing titles along the way, leading up to the 1960 Olympics in Rome, where he took home the gold medal as the light heavyweight champion. Ali turned professional shortly after the Olympics. His knack for talking up his talents and skills in a showboating style earned him the nickname "the Louisville Lip." But Ali would always put his money where his mouth is, backing up his talk with action. That became evident in 1964, when Ali got his opportunity at the title in a highly anticipated fight in Miami, Florida against heavyweight champion Sonny Liston. Ali broke out with a series of poetic insults and rhymes, including his iconic "Float like a butterfly, sting like a bee." And he stunned the champ in short order, beating the favored Liston with a sixth-round technical knockout (TKO).

After the fight, Ali famously proclaimed: "I am the greatest! I am the greatest! I'm the king of the world."

All-time champion

In the years following, Ali transformed from the tough-talking, braggadocious boxer into a powerful, fleet-footed champion, arguably considered the greatest heavyweight boxer of all time. That culminated with a trilogy of fights with legendary champion Joe Frazier, ending with the third and most talked-about match of the century in 1975, known as the "Thrilla in Manila."

Ali and Frazier exerted their power and might, trading blow-for-blow in an epic match that saw Ali triumph over Frazier with a TKO at the end of the 14th round. Ali admitted after the fight that he was spent, saying it "was the closest thing to dying that I know" and calling Frazier "the greatest fighter of all times next to me." "When he beat Joe Frazier in their third fight," Roach told Anadolu Agency (AA), referring to the fight that stood out most to him during Ali’s career, "he took a left hook that would have knocked out anyone else." Roach knows a thing or two about boxing champions, having trained former world champion boxer Manny Pacquiao, the only eight-division world champion in the history of boxing who won 12 major world titles.

Roach says Ali was his inspiration. "I built my gym in Hollywood, California, Wild Card Boxing Club, thinking maybe the next Muhammad Ali would walk through the door," said Roach. "Then one day Manny Pacquiao walked in. Manny was my Muhammad Ali." Roach says Ali’s influence on both himself and his boxers was seen far and wide. "His personality and his boxing. When he came to my gym, years after he had retired, he stayed for hours, doing magic tricks, shadow boxing, hitting the bag," said Roach. "We were enthralled."

"He believed in his greatness," said former professional boxer and current world champion trainer Joel Diaz. "What made Ali the greatest fighter of all time was his character, his boxing ability," Diaz told AA. "He was a different fighter, a different breed. He had a lot of confidence in himself. He basically got in the ring with all the confidence in the world. His footwork and his mentality made him the greatest fighter of all time." Diaz knows firsthand how that confidence translates into success. He currently trains undefeated World Boxing Association (WBA) champion Dmitry Bivol, who in May defeated the best-known boxer in the world, Canelo Alvarez, to earn the light-heavyweight title. "Muhammad Ali was always an inspiration ... his character and the way he presented himself ... he was always positive," said Diaz. "A fighter that believes in himself in everything he does, and is positive ... obviously, he’s a motivation,"

Ali, however, was not just an inspiration in the ring. As his career blossomed, he became one of the most high-profile celebrities in the world, championing civil rights and social justice causes. When black people were refused seats in the fronts of buses and denied entry to restaurants, he renounced his birth name of Cassius Clay as his "slave name" and found guidance through the Nation of Islam. Ali’s mission was to reject the pacifism of most civil rights activism of that time and empower the African American community against white racism. His public conversion to Islam in 1964 was a defining moment in his life. Elijah Muhammad, the Nation of Islam leader, renamed him Muhammad Ali, meaning one who is worthy of most high praise.

Many of his critics continued to call him by his birth name, Cassius Clay, but that did not deter Ali from embracing his Muslim faith and beliefs. "I am America. I am the part you won't recognize," said Ali to his detractors. "But get used to me. Black, confident, cocky; my name, not yours; my religion, not yours; my goals, my own; get used to me."

Ali also refused to serve in the Vietnam War in 1967 due to his religious beliefs, saying at the time, "No, I’m not going 10,000 miles from home to help murder and burn another poor nation simply to continue the domination of white slave masters of the darker people the world over." Ali was found guilty of draft evasion, sentenced to five years in prison and stripped of his heavyweight title. His conviction was eventually overturned, but he was banned from boxing for three years for taking such a bold stance.

Ali was an anti-establishment figure who transcended race, religion and every other barrier that kept the world divided. His battles outside the ring only complemented and highlighted his fights inside the ring. "His greatness was his ability to connect with people," said Roach. "Muhammad Ali was the face of boxing, and in a lot of ways he still is."