That is a phrase from the Anne of Green Gables series, and it means the same thing as Anne's phrase "kindred spirits". Either someone knows Joseph, or they don't. And when you meet 'em, it usually takes no more than five minutes to tell if they know Joseph, too.
This past weekend, I had dinner with a new person who does not know Joseph. And I met one new person who definitely does. Welcome, Marc!
The thing I like about the race that knows Joseph is that sometimes family members belong - and sometimes they don't. It's not their fault, it's just is how it is. But how fun is it when someone in your family IS a kindred spirit?
Even if there is a dearth of kindred spirits in the family, that makes the others all the more precious. I always wanted a sister. And one has made her way to me. She definitely is of the race that knows Joseph and just like getting to know any sibling worth their salt, it's a slow and patient process that'll bloom in its own time.