To be a Rastafarian, one must start by believing that Marcus-I was a prophet, a black prophet, that what he prophesied was true, in particular his prophecies about the black race and the black King,
and then move on to Emmanuel-I and Selassie-I, seen?
One must accept this Trinity as the manifestation of the return of Christ, in order for them to become a Rastafarian, seen?
So when the prophet Marcus-I told the black people that they would soon be saved from captivity by a black King who would be crowned in Africa, all the people who heard him and who held some spirituality set their eyes vigilantly towards Africa awaiting the promised King.
This marked the beginning of the Rastafari Movement...just that small congregation of those people who awaited this King was enough to start the movement, seen?
So these believers who waited for the coronation of that black King had already proclaimed that this would be the return of Christ, to save His people from Babylon.
Thus Haile Selassie-I was regarded by those people as the return of Christ before His coronation, but soon after Marcus-I had promised the people a Saviour, seen?
So in 1930, when these people received the news that a black King had been crowned in Africa who is from the line of David, they burst into great joy because the words of the black Prophet had been fulfilled.........so they knew this was the promised Messiah!
That is how Rastafarians began worshipping King Selassie-I, seen?
And to answer your question fully, I will respond with the stardard definition of a Rastafarian;
A Rastafarian is a person who holds that blacks are the chosen people, that Haile Selassie-I is the second coming of Christ, and that blacks shall eventually return to their Africa, seen?
So you can't be regarded as a Rastafarian if you reject Selassie-I as the reincarnation of Christ.