What shocking thing does Montag do that horrifies Faber,what is the last refuge for "dangerous intellectual" in Fahrenheit 451?
Faber has essentially given up on the world; he cares so much about it that it makes him physically ill to realize that this society is inevitably headed for self-destruction. Montag agrees that an apocalypse is coming, but thinks that there must be people prepared to salvage what's left of human civilization in the aftermath. When Faber refuses to help him do this, protesting that there's no point to it, Montag begins tearing pages out of a Bible, presumably because he thinks it will spur Faber to act.
The last refuge for the dangerous intellectual is the stock market, which is how Faber got the money to finance his life and creations, such as the two-way radio. This seems to be a bit of a throwaway comment, or a remark on Faber's eccentricity, and not really a deep insight that the novel means to present as part of its message. I think this question was asked mostly to see if an assigned reading had been completed.
You can find this stuff in the part of the book where Montag goes over to Faber's house for the first time.
What Montag does that horrifies Faber is to start ripping up the Bible. Faber is horrified because that is, as far as he knows, the only copy of the Bible around and he doesn't want it destroyed.
The last refuge for a dangerous intellectual is the stock market. That is how Faber has gotten himself some money even though he has no job. He has played the stock market and gotten enough money to use to invent the "bullet" that he gives Montag.