Steve Nash Bar Mitzvah Bash -- The BILLIONAIRE Connection

We finally found out why NBA superstar (and non-Jewish person) Steve Nash made an appearance at Bar Mitzvah in L.A. last weekend ... and it was all made possible thanks to one of the RICHEST men on the planet.

Turns out, the boy who became a man is 13-year-old Mike Feinberg -- and his daddy Jeff is a very wealthy person.

How wealthy, you ask? Well, Mr. Jeffrey Feinberg is a big-shot hedge fund manager who worked as an executive at Soros Fund Management -- as in, billionaire George Soros ... the wealthiest hedge fund manager in the world.

After a couple of years kicking ass and raking in billions for the hedge fund, Jeff branched off and ran his own investment company -- and it seems he did pretty well, 'cause back in 2009, he was reportedly in the hunt to buy a stake in the Brooklyn Nets.

Sources close to Feinberg tell us ... Jeff is a huge basketball fan with a TON of NBA friends -- including Mr. Nash.

Jeff's wife Stacey ain't exactly a nobody either ... she's the daughter of legendary sports agent Bob Woolf ... whose client list included Larry Bird, Doug Flutie, John Havlicek and even Larry King.