user-1: 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?


user43: Buddhist is someone who TRIES (correctly or incorrectly, perfectly or imperfectly) to follow the teaching of Buddha and its interpretations by the Buddha's disciples.

user471: The term "Buddhist" in English applies to just about anyone who associates himself or herself with the broad religion and culture of Buddhism, with all its schools and traditions.
For e.g. it is common in Buddhist majority and minority countries, for a person to call himself "Buddhist" but know nothing much of the teachings and simply visit the Buddhist temple to light joss sticks and get a blessing on Vesak Day. Please see this answer for an example of keys, amulets or other personal objects being blessed by a monk, who chants on them.
By this measure, even Devadatta in the time of the Buddha would be considered a "Buddhist".
Of course, what the Buddha himself thought and taught, can be found in MN 70.
There are people doing it the right way and the wrong way:

And how is enlightenment achieved by gradual training, progress, and
practice? It’s when someone in whom faith has arisen approaches a
teacher. They pay homage, lend an ear, hear the teachings, remember
the teachings, reflect on their meaning, and accept them after
consideration. Then enthusiasm springs up; they make an effort, weigh
up, and persevere. Persevering, they directly realize the ultimate
truth, and see it with penetrating wisdom.
Mendicants, there has not been that faith, that approaching, that
paying homage, that listening, that hearing the teachings, that
remembering the teachings, that reflecting on their meaning, that
acceptance after consideration, that enthusiasm, that making an
effort, that weighing up, or that striving. You’ve lost the way,
mendicants! You’re practicing the wrong way! Just how far have these
foolish people strayed from this teaching and training!

And people who are doing it the right way, can be classified into seven types:

Mendicants, these seven people are found in the world. What seven? One
freed both ways, one freed by wisdom, a personal witness, one attained
to view, one freed by faith, a follower of the teachings, and a
follower by faith.

The details are given in the sutta.

user13375: The word Buddhist is a convention. All words are mere conventions. They have no inherent existence. They are subject to change over time. In fact, it would be an impossible mode of existence for them not to change over time. They are not fixed and rigid things endowed with an inherent meaning in some impossible platonic existence. It is not possible for the word Buddhist - or any word - to be fixed in time with a given definition perfectly understood by all.
Now, this does not mean it doesn't have good or bad definitions. Again, like all words, different people at different times will string together other words and sentences in attempt to define what their understanding is of a given word. Others will come along and dispute this or that part as erroneous, superfluous, or missing some vital ingredient central to their understanding of some given word. There is not some objective and valid answer to the definition of some given word that is bequeathed by the universe at some given time that all must agree to as the correct and uniform definition. To think that there is is quite a misunderstanding of the nature of how words exist.
Of course, this doesn't mean that it is possible to go around claiming no words have any definitions whatsoever since I can call an orange an elephant if I want and no one can say I'm objectively wrong by some universal law handed down by the gods from on high regarding the definition of oranges and elephants. That is also a gross misunderstanding of the nature of how words exist.
So all that said... what is the definition of a Buddhist? I take my definition from Buddhism: One Teacher, Many Traditions which has this to say:

All Buddhist traditions agree that taking refuge in the Three Jewels is the demarcation of becoming a Buddhist.

Which I think is both true and a pretty handy and useful definition.
To your more detailed questions:

If he was a Buddhist; and a Buddhist can believe & practice
incorrectly; are all humans Buddhists?

Of course not. See the above definition which pretty easily leads to the conclusion that not all humans are Buddhists which is of course what we'd expect in common language.

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?

Huh? Why do you think that a Buddhist can't believe wrong things?? Anyway, that's not part of the above definition and I'd say inline with what we'd expect with common language so again a win for the above definition.
Hope this helps!

-1: <p>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.</p>
<ul>
<li>Devadatta did not understand dependent origination correctly; essence of the teacher's message.</li>
</ul>
<p>If he did he would've been ariya; ariya savaka; 'a learner in training and not liable to go to hell.</p>
<p>Consider this</p>
<p>In Islam, a person who doesn't understand the essence of the prophet's message is considered an outsider.</p>
<p>A muslim belives that there is a god. Whether he is a good or a bad muslim doesn't matter.</p>
<p>If a person doesn't believe in god, holds the view that there is no god, that person is an outsider to Islam.</p>
<p>I am using Islam as just an example.</p>
<p>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.</p>
<p>So was Devadatta a Buddhist?</p>
<ul>
<li>If he was a Buddhist; and a Buddhist can believe &amp; practice incorrectly; are all humans Buddhists?</li>
<li>If he wasn't a Buddhist; and a Buddhist can't belive wrong things &amp; practice incorrectly; then why do you call ie Mahayana and Theravada followers Buddhists when seeing that they believe different things and pracrice differently?</li>
</ul>
 43: <p>Buddhist is someone who TRIES (correctly or incorrectly, perfectly or imperfectly) to follow the teaching of Buddha and its interpretations by the Buddha's disciples.</p>
 471: <p>The term &quot;Buddhist&quot; in English applies to just about anyone who associates himself or herself with the broad religion and culture of Buddhism, with all its schools and traditions.</p>
<p>For e.g. it is common in Buddhist majority and minority countries, for a person to call himself &quot;Buddhist&quot; but know nothing much of the teachings and simply visit the Buddhist temple to light joss sticks and get a blessing on Vesak Day. Please see <a href="https://buddhism.stackexchange.com/a/9172/471">this answer</a> for an example of keys, amulets or other personal objects being blessed by a monk, who chants on them.</p>
<p>By this measure, even Devadatta in the time of the Buddha would be considered a &quot;Buddhist&quot;.</p>
<p>Of course, what the Buddha himself thought and taught, can be found in <a href="https://suttacentral.net/mn70/en/sujato" rel="nofollow noreferrer">MN 70</a>.</p>
<p>There are people doing it the right way and the wrong way:</p>
<blockquote>
<p>And how is enlightenment achieved by gradual training, progress, and
practice? It’s when someone in whom faith has arisen approaches a
teacher. They pay homage, lend an ear, hear the teachings, remember
the teachings, reflect on their meaning, and accept them after
consideration. Then enthusiasm springs up; they make an effort, weigh
up, and persevere. Persevering, they directly realize the ultimate
truth, and see it with penetrating wisdom.</p>
<p>Mendicants, there has not been that faith, that approaching, that
paying homage, that listening, that hearing the teachings, that
remembering the teachings, that reflecting on their meaning, that
acceptance after consideration, that enthusiasm, that making an
effort, that weighing up, or that striving. You’ve lost the way,
mendicants! You’re practicing the wrong way! Just how far have these
foolish people strayed from this teaching and training!</p>
</blockquote>
<p>And people who are doing it the right way, can be classified into seven types:</p>
<blockquote>
<p>Mendicants, these seven people are found in the world. What seven? One
freed both ways, one freed by wisdom, a personal witness, one attained
to view, one freed by faith, a follower of the teachings, and a
follower by faith.</p>
</blockquote>
<p>The details are given in the sutta.</p>
 13375: <p>The word Buddhist is a convention. All words are mere conventions. They have no inherent existence. They are subject to change over time. In fact, it would be an impossible mode of existence for them <em>not</em> to change over time. They are not fixed and rigid things endowed with an inherent meaning in some impossible platonic existence. It is not possible for the word Buddhist - <strong>or any word</strong> - to be fixed in time with a given definition perfectly understood by all.</p>
<p>Now, this does not mean it doesn't have good or bad definitions. Again, like all words, different people at different times will string together <em>other words</em> and sentences in attempt to define what their understanding is of a given word. Others will come along and dispute this or that part as erroneous, superfluous, or missing some vital ingredient central to their understanding of some given word. There is not some objective and valid answer to the definition of some given word that is bequeathed by the universe at some given time that all must agree to as the correct and uniform definition. To think that there is is quite a misunderstanding of the nature of how words exist.</p>
<p>Of course, this doesn't mean that it is possible to go around claiming no words have any definitions whatsoever since I can call an orange an elephant if I want and no one can say I'm objectively wrong by some universal law handed down by the gods from on high regarding the definition of oranges and elephants. That is also a gross misunderstanding of the nature of how words exist.</p>
<p>So all that said... what is the definition of a Buddhist? I take my definition from <a href="https://rads.stackoverflow.com/amzn/click/com/1536617563" rel="nofollow noreferrer" rel="nofollow noreferrer">Buddhism: One Teacher, Many Traditions</a> which has this to say:</p>
<blockquote>
<p>All Buddhist traditions agree that taking refuge in the Three Jewels is the demarcation of becoming a Buddhist.</p>
</blockquote>
<p>Which I think is both true and a pretty handy and useful definition.</p>
<p>To your more detailed questions:</p>
<ul>
<li>If he was a Buddhist; and a Buddhist can believe &amp; practice
incorrectly; are all humans Buddhists?</li>
</ul>
<p>Of course not. See the above definition which pretty easily leads to the conclusion that not all humans are Buddhists which is of course what we'd expect in common language.</p>
<ul>
<li>If he wasn't a Buddhist; and a Buddhist can't belive wrong things &amp; practice incorrectly; then why do you call ie Mahayana and Theravada followers Buddhists when seeing that they believe different things and pracrice differently?</li>
</ul>
<p>Huh? Why do you think that a Buddhist can't believe wrong things?? Anyway, that's not part of the above definition and I'd say inline with what we'd expect with common language so again a win for the above definition.</p>
<p>Hope this helps!</p>
