"I get that atheists are often exeedingly moral. Some of the worst immoral acts I have personally seen were commited by exceedingly fundamentalist evangelicals who were formally known as Jews for Jesus."

Scott Roeder who was convicted of murdering Dr. Tiller was part of this group. They profess wanting to protect the "unborn" but don't give a damn for actual human life.
Operation Rescue is a terrorist organization.

The joke is that Jews, by in large, think that Jews for Jesus is an abomination. It runs counter to their belief system.