Though Muhammad Ali Clay or Cassius Marcellus Clay was raised as a Baptist, he believes that all religions are good and right. According to him, when Jesus Christ came as a prophet, he was good and right, but, unfortunately, Christians do not practice or live Christianity. They are Christians by name only because they do not really follow this religion.
He chose Islam because he saw much love from Muslims. As a Christian, he was not done good. According to him, whites are always good, whereas blacks are always bad. See this video to know why Muhammad Ali Clay reverted to Islam and how he became one of the most famous Muslim preachers!
May God show him mercy!!