> Karl Popper's 'Paradox of Tolerance' applies. TL,DR: tolerating intolerance turns out to be a bad idea.
To provide some additional context to an often over-(ab)used quote:
I often see it used as a "thought-terminating cliché". Applying it this way would likely meet his definition of intolerant at least half-way:
Popper defines what he means by 'intolerance'. According to his definition, it requires both (A) the refusal to participate in 'rational discourse', and (B) incitement to and use of violence against people with different views.
You will find 'intolerant people' on all sides of the political spectra. (I don't see how dumbing it down to 'left' and 'right' really serves any rational discourse.)
> "I do not imply, for instance,
that we should always suppress the utterance of intolerant philosophies ; as long as we can counter them by rational argument and keep them in check by public opinion, suppression would certainly be most unwise. But we should claim the right to suppress them if necessary even by force ; for it may easily turn out that they are not prepared to meet us on the level of rational argument, but begin by denouncing all argument ; they may forbid their followers to listen to rational argument, because it is deceptive, and teach them to answer arguments by the use of their fists or pistols. We should therefore claim, in the name of tolerance, the right not to tolerate the intolerant. We should claim that any movement preaching intolerance places itself outside the law, and we should consider incitement to intolerance and persecution as criminal, in the same way as we should consider incitement to murder, or to kidnapping, or to the revival of the slave trade, as criminal." (The Open Society and Its Enemies, 1945)
https://ia800100.us.archive.org/10/items/in.ernet.dli.2015.5...
(It's well worth to read as a whole, given how often it is used and abused out of its surrounding context in the book.)
> "Conscience could be defined as the intuitive capacity of man to find out the meaning of a situation. Since this meaning is something unique, it does not fall under a general law, and an intuitive capacity such as conscience is the only means to seize hold of meaning Gestalts. […] True conscience has nothing to do with what I would term “superegotistic pseudomorality.” Nor can it be dismissed as a conditioning process. Conscience is a definitely human phenomenon. But we must add that it is also “just” a human phenomenon. It is subject to the human condition in that it is stamped by the finiteness of man. For he is not only guided by conscience in his search for meaning, he is sometimes misled by it as well. Unless he is a perfectionist, he also will accept this fallibility of conscience. It is true, man is free and responsible. But his freedom is finite. Human freedom is not omnipotence. Nor is human wisdom omniscience, and this holds for both cognition and conscience. One never knows whether or not it is the true meaning to which he is committed. And he will not know it even on his deathbed. Ignoramus et ignorabimus—we do not, and shall never know—as Emil Du Bois-Reymond once put it, albeit in a wholly different context of the psychophysical problem. But if man is not to contradict his own humanness, he has to obey his conscience unconditionally, even though he is aware of the possibility of error. I would say that the possibility of error does not dispense him from the necessity of trial. As Gordon W. Allport puts it, “we can be at one and the same time half-sure and whole-hearted. *The possibility that my conscience errs implies the possibility that another one’s conscience is right. This entails humility and modesty. If I am to search for meaning, I have to be certain that there is meaning. If, on the other hand, I cannot be certain that I will also find it, I must be tolerant.* This does not imply by any means any sort of indifferentism. Being tolerant does not mean that I share another one’s belief. But it does mean that I acknowledge another one’s right to believe, and obey, his own conscience. […] Suffering is only one aspect of what I call “the tragic triad” of human existence. This triad is made up of pain, guilt, and death. There is no human being who may say that he has not failed, that he does not suffer, and that he will not die." (Viktor Frankl, The Will to Meaning, 1972)
> "For tolerance, rightly understood, has not the slightest thing to do with indifferentism. And if we finally ask ourselves: how can I, being one hundred percent convinced of my own faith, possibly accept another's faith, another's conviction? Do I not, by that very act, become unfaithful to my own faith and my own conviction? We must answer this question in the negative. For I do not respect another's faith because I can share it, but because I must respect the other person himself. Note: Tolerance does not consist in sharing another's view, but only in granting the other the right to be of a different view at all. On the other hand, tolerance is also misunderstood if one goes so far as to grant the other the right to be, for his own part, intolerant." (machine translated from the German original)