Dylan Matthews explains on Vox:

On the second night of the 2016 Republican Convention, primetime speaker Ben Carson warned attendees and viewers of the worst thing about Hillary Clinton: her ties to famed community organizer Saul Alinsky.

“One of the things that I have learned about Hillary Clinton is that one of her heroes, her mentors, was Saul Alinsky,” Carson declared as the crowd booed. “Her senior thesis was about Saul Alinsky … This was someone she greatly admired and that affected all of her philosophy subsequently.” And why is this bad? Well, Carson explained (erroneously), Alinsky dedicated his book Rules for Radicals to none other than … Satan himself!

“This is a nation where our Pledge of Allegiance says we are one nation under god. This is a nation were every coin in our pocket and every bill in our wallets says ‘In god we trust.’ So, are we willing to elect someone as president who has, as their role model, somebody who acknowledges Lucifer?” Carson asked. Again, the crowd booed.

The rapturous response Carson got was understandable given villainous reputation Alinsky has earned by now in certain conservative circles. But for the vast majority of people who’ve never heard of Alinsky, the speech was baffling.

So: who is this guy, and why does he matter?