The core of the joke is a reference to Blackjack, where you're dealt two cards at the start and the goal is to try and have the highest/closest value on your cards to equal 21 without going over, and in the least number of cards.
If you're dealt an 11, there's no reason not to "hit" for another card, since you can't go over. At 16 the odds are much less in your favor, since you're low enough to not likely have the closest hand, but high enough that getting another card will probably push you over 21, which means taking a risk.
So if dealt the bad hand they've been given, instead of folding, just commit and have faith that the extra cards will give them a better hand.
As for the Jews, I believe he's telling them to take a leap of faith in believing in Christ, since they'd be arguably closer to it than an unbeliever (with an 11), but simultaneously far enough away to probably be wrong and lose (with a 16).