greswd said:
do you know what happened to all the mathematicians who opposed Cantor? when did his ideas get mainstream acceptance?
About those opposed to Cantorian set theory, I gave you a (partial?) answer in post #35; do you have any further questions about that?
As for when Cantor's ideas became mainstream, I'd say probably in the 30's, when it became clear the limits of logic set theory in general (due to Gödel's theorems). One nice indication of this shift is Tarski's 1935 postscript to his 1933 paper on truth. In the 1933 paper, Tarski still worked inside the framework of (roughly Russellian) type theory, with the restrictions imposed by that (noticed that the 1933 paper was actually written during 1929-1931, so
before Gödel published his results). In the postscript, however, he indicates that he now prefers to work inside a framework which accepts the transfinite hierarchy and which is thus much closer to Cantorian set-theory. This is indicative that in the early to mid 30's, Cantorian set theory, formalized as a first-order theory, became much more mainstream.
What occasioned this shift? There are a number of factors here. Certainly, one important factor was the recognition of first-order logic as an ideal framework for such theories. Today, it's common to present ZFC in a first-order theory, so that we often forget the fact that, when Zermelo proposed his axioms in 1908, his preferred framework was
second-order logic----indeed, until Weyl's and Hilbert's work in the 1910's, almost nobody saw a clear distinction between first and second order logic (take a look, e.g., at Frege's
Begriffsschrift from 1879, where there's no distinction between first and second order instantiation). Even Skolem's defense of first-order logic in the early 1920's was met with skepticism. So it seems that it was only after Gödel's groundbreaking work that first-order gained in prominence, which also paved the way for a more systematic investigation of Cantorian set theory.
Other important factors were the results of investigations, by Tarski, von Neumann, Zermelo, and others, into the consequences of adopting a Cantorian framework. For instance, we now take for granted the von Neumann ordinals and transfinite recursion, but before von Neumann formulated these in the 1920's, there wasn't any rigorous theory of the ordinals and their properties. Similarly, the discovery by Tarski and others of several equivalents to the Axiom of Choice certainly made it easier for others to swallow it, specially when it was discovered that many who opposed it actually employed it implicitly in their work. Relatedly,
classical descriptive set theory was just being born, with the impressive results by Lebsegue, Lusin, Suslin, and others on analytic and Borel sets. Together, these developments established set theory as an important tool in the investigation of classical mathematical areas, such as analysis and topology.
Finally, I'd also add that Gödel's work on the independence of the Continuum Hypothesis and the Axiom of Choice from ZFC (and other theories such as NBG) were also crucial in establishing set theory as an important area of investigation in its own right. They showed that set theory was not only a rich tool to be used, but also a field with its own questions and research program (e.g. inner model theory). So, by 1939, when Gödel announced his results, I'd say that Cantorian set theory was firmly established as a solid and mature research program. Considering how young it was by then, that's certainly an impressive feat!
Incidentally, if you're interested in pursuing these issues in depth, I'd strongly recommend that you read José Ferreirós's
Labyrinths of Thought, a very detailed and comprehensive account of the development of set theory.