In another post, he wrote: “You like to bring in hostile invaders to dwell among us? We appreciate the list of friends you have provided.” Alongside it was a link to information on the National Refugee Shabbat Event, celebrated on Oct. 20 at more than 300 Jewish congregations in 33 states.

Mark Hetfield, the president of HIAS, said the climate of political rancor over immigration gave impetus to the national refugee event. “We felt that at a time when the U.S. is doing less and less for refugees, that we must demonstrate, as a refugee people, that it is more important than ever to continue to welcome refugees as a community,” he said.

At a gathering at Congregation Shir Hamaolot in Irvine, Calif., Jackie Menter, a co-founder of the Orange County Jewish Coalition for Refugees, shared her experience working with Middle Eastern and African refugees on the Greek island of Chios last year.

She spoke of a Syrian girl who had watched a land mine kill her mother as they were fleeing captivity by the militant group Islamic State, and a man named Mustafa, who returned home to his apartment in the Syrian city of Aleppo to find the entire building bombed out and his wife, children and mother dead.

Citing biblical passages about welcoming the stranger, Ms. Menter urged congregants to get involved with HIAS’s efforts to support refugees and asylum seekers. On Saturday night, after the attack on the synagogue in Pittsburgh, she said, “Evil, hatred and racism will not keep justice and mercy from prevailing.”

The Trump administration has sharply reduced the number of people fleeing violence and persecution who are admitted to the United States as refugees, lowering the annual ceiling from the 110,000 set before Mr. Trump took office to 45,000 in the 2018 fiscal year, which ended Sept. 30, and to 30,000 for fiscal 2019. Delays caused by extra levels of screening ordered by the Trump administration — targeting especially people from 11 countries, 10 of them predominantly Muslim — meant that only 22,491 refugees actually arrived in fiscal 2018.