The conversion of Ali to Islam was the result
of his freindship with the “The Prince” Malcolm
X .
While growing up Ali developed huge anger for
“ the white “ community because of the racial
discrimination he and the Black community
have been facing .He wanted to denounce
everything that have been forced onto them by
the white including their faith and the name “
Clay”,
He met Malcolm X somewhere around 1962
and was highly influenced by his teaching and
belief in “Black Supremecy”.
Malcolm saw in Clay a charisma that drew
people to him. He began to cultivate the
friendship, partly because Malcolm hoped he
could draw Clay into the Nation; the young
boxer could potentially bring admirers into the
fold with him, a benefit for the Nation — and
for Malcolm.
ALi converted to his new faith even before
winning his world championship but kept it
quiet with the fear that "he will never get a
shot at the championship."
He was introduced to the faith by another
great, the Malcolm X who was a Nation of
Islam member but left it for sunni Islam and
Ali followed his spiritual mentor by following
sunni Islam.

https://www.quora.com/Why-did-Muhammad-Ali-convert-to-Islam

The slave masters whom he loathed were christians. They named him cassius clay, it won't make sense to remain with them when in dissagrement would it? Note it had nothing to do with a superiority or inferiority of religions

Mtcheeeww