Philosopher Karl Popper defined the paradox in 1945 in The Open Society and Its Enemies Vol. 1

“Less well known is the paradox of tolerance: Unlimited tolerance must lead to the disappearance of tolerance. If we extend unlimited tolerance even to those who are intolerant, if we are not prepared to defend a tolerant society against the onslaught of the intolerant, then the tolerant will be destroyed, and tolerance with them.”

He concluded that we are warranted in refusing to tolerate intolerance: “We should therefore claim, in the name of tolerance, the right not to tolerate the intolerant.” SOURCE