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

A buddhist is someone who keeps the five precept and takes refuge in the Buddha, Dharma and Sangha.

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

This is a question that can have various answers depending on how one might define what a "Buddhist" is or what makes one a "Buddhist" in the first place. To avoid that altogether, let's forget about the word "Buddhist" for now as it did not exist at the time of the Buddha and let us look to the Tipitaka...

Then the Blessed One, having understood Brahma's invitation, out of compassion for beings, surveyed the world with the eye of an Awakened One. As he did so, he saw beings with little dust in their eyes and those with much, those with keen faculties and those with dull, those with good attributes and those with bad, those easy to teach and those hard, some of them seeing disgrace and danger in the other world. Just as in a pond of blue or red or white lotuses, some lotuses — born and growing in the water — might flourish while immersed in the water, without rising up from the water; some might stand at an even level with the water; while some might rise up from the water and stand without being smeared by the water — so too, surveying the world with the eye of an Awakened One, the Blessed One saw beings with little dust in their eyes and those with much, those with keen faculties and those with dull, those with good attributes and those with bad, those easy to teach and those hard, some of them seeing disgrace and danger in the other world.

Having seen this, he answered Brahma Sahampati in verse:

Open are the doors to the Deathless to those with ears. Let them show their conviction. Perceiving trouble, O Brahma, I did not tell people the refined, sublime Dhamma.

Then Brahma Sahampati, thinking, "The Blessed One has given his consent to teach the Dhamma," bowed down to the Blessed One and, circling him on the right, disappeared right there.

SN 6.1

We see here that the Buddha saw that people could understand the Dhamma, regardless of if it would be easier or harder for them to do so. In seeing this, he decided to teach it (obviously). Let's focus on the last verse. He is essentially saying that people can attain Nibbana as he, but only to "those with ears," meaning only those who correctly hear the path can attain Nibbana. If the path was incorrectly heard, one could stray from it and thus never reach the Further Shore.

Therefore, people can hear (and subsequently follow) the path incorrectly, but will never be able to attain the goal as a result and people can hear (and subsequently follow) the path correctly, but will attain the goal as a result. Fast forward to when the word "Buddhist" was developed and you just replace the word "people" in the previous sentence with "a Buddhist."

Thus, a Buddhist and what makes a person a Buddhist is if they are following the Dhamma. Now if the Dhamma they hear (and subsequently follow) is correct or incorrect, that is another story, but in short, one can come to know if it is correct by direct experience to verify it (per the Kalama Sutta).

In my opinion, one should not worry as to what a Buddhist is or what makes a person a Buddhist, it all depends on if they are following the Dhamma, not what they are identifying as. What one should worry about is if the Dhamma they hear is correct and will lead them to reach the Further Shore (which they can come to know through their experience).

So was Devadatta a Buddhist?

Devadatta heard (and subsequently followed) the Dhamma correctly as the Buddha himself expounded it to him in the first place (as far as I understand). The problem is that he took what he heard and distorted it. When he did that, he was not following the Dhamma, he was following his own views.

In that sense, he was following the Dhamma until he swayed from that path and onto his own distorted one, making him a Buddhist at one point and not one at another point.

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

Devadatta was a Buddhist until he stopped following the Dhamma for his own distorted views. One would not be a Buddhist if one is not practicing the Dhamma that the Buddha expounded. All humans are not Buddhists because all humans do not follow the Dhamma, only a percentage of them do. Now whether or not the Dhamma one follows is correct or incorrect, that is another story, but (again) in short, one can come to know if it is correct by direct experience to verify it.

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?

Refer to my answer in the previous question.

I hope all of this helps. May you be well.

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.

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.

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!