In most American cities, Jews tend to live in the suburbs. Not so in Pittsburgh, where more than half of the Jewish community still lives in the city, mainly in Squirrel Hill. And unlike in other urban centers, like Los Angeles or New York, Pittsburgh’s Jewish community is small enough that we don’t stay in our religious and political lanes. When I spoke last weekend at my grandparents’ Reform synagogue in my hometown, there were liberal and conservative, Reform and Orthodox, American and Israeli-born Jews who waited to hug me — and argue with me — after. That’s Pittsburgh.

There is a phrase in the Talmud that has always felt especially relevant to our community: Kol Yisrael Arevim Zeh Bazeh. All of Israel is responsible for one another. For us that is not a lovely theory but a lived reality.

As with many synagogues in America, the doors to Tree of Life and Pittsburgh’s other shuls on Saturday mornings did not have any security and were open to all comers. We live according to our values — the ones that the accused appears to despise.

One of the alleged shooter’s obsessions on social media was HIAS, the Hebrew Immigrant Aid Society, a Jewish organization originally founded in the late 1800s to resettle Jews fleeing pogroms in Eastern Europe. Today it rescues Jews and non-Jews facing persecution all over the world.

A few weeks ago, Mr. Bowers shared a link to an event called Refugee Shabbat, a national initiative organized by HIAS, of which Tree of Life was a participating synagogue. “Why hello there HIAS! You like to bring in hostile invaders to dwell among us?” he wrote on a social networking site often used by alt-right activists and white nationalists.

Just this morning, he posted: “HIAS likes to bring in invaders that kill our people. I can’t sit by and watch my people get slaughtered. Screw your optics, I’m going in.”

The heartbreaking coincidence is that the Jewish emphasis on the open door, on welcoming the stranger, is exactly what the Jews of Tree of Life and the Jews of every synagogue big and small in every far-flung corner of the globe were reading about this Shabbat morning.