So what is a Buddhist and what makes a person a Buddhist?

In brief Devadatta was Buddha's cousin and a monk. He could fly, tried to kill the Buddha, did many bad things and went to hell. * Devadatta did not understand dependent origination correctly; essence of the teacher's message. If he did he would've been ariya; ariya savaka; 'a learner in training and not liable to go to hell. Consider this In Islam, a person who doesn't understand the essence of the prophet's message is considered an outsider. A muslim belives that there is a god. Whether he is a good or a bad muslim doesn't matter. If a person doesn't believe in god, holds the view that there is no god, that person is an outsider to Islam. I am using Islam as just an example. Buddhism is very different in this regard. It is the only(?) religious designation which doesn't require conformity in belief and is some abomination in that regard. So was Devadatta a Buddhist? * If he was a Buddhist; and a Buddhist can believe & practice incorrectly; are all humans Buddhists? * If he wasn't a Buddhist; and a Buddhist can't belive wrong things & practice incorrectly; then why do you call ie Mahayana and Theravada followers Buddhists when seeing that they believe different things and pracrice differently?