Christian antipathy towards the Jews began in the first few centuries of Christendom. It became widespread and violent during the Crusades in the 13th century, even more violent during the pogroms of the late 1800s, and then, just when it seemed it could not get worse, resulted in the murder of millions of Jews at the hands of baptized Christians in the Holocaust, while church leaders looked on.

Only then did the churches begin to come to their senses. In 1965, Pope Paul VI declared in his “Nostra Aetate” that “from enemies and strangers, we [Christians and Jews] have become friends and brothers.” It seemed that even if the “Christ killer” and blood libel myths had not entirely become things of the past, they would no longer result in the murder of Jews.

And then came Poway.

The shooting at the Chabad synagogue in Poway, Calif., on the last day of Passover was racially but also theologically driven. It was a most terrible flash point in the history of Jewish-Christian relations. Once again, the dead and wounded are victims of Christian anti-Semitism. In the shooter’s own letter, posted online before the shooting, there was a clear and unequivocal Christian call to action: “To my brothers in Christ … although the Jew who is inspired by demons and Satan will attempt to corrupt your soul with the sin and perversion he spews, remember that you are secure in Christ.”

If ever words could kill. This letter epitomizes the link between ideas and actions.

The shooter describes himself as having “a loving family” and “great friends.” He was training to become a nurse. He attended church. He was also very familiar with the history of anti-Semitism. Not that he had any sympathy. In fact, he accuses Jews of harming Christians, from the killing of St. Stephen in 34 C.E., to Simon of Trent in the 15th century, through to the recent deaths of Christians in Syria.

We are familiar with neo-Nazi white supremacists, including the Pittsburgh killer, and Islamist killers such as the shooter in Toulouse, France, in March 2012. But we are not so familiar with white supremacists who are not a part of a radical organization, who go to church on Sunday, are training to become nurses—and who want to kill Jews.

As a trained theologian and Holocaust historian, reading the killer’s letter gave me the deepest sense of disquiet I can remember in my adult life. This was an entirely rationalized killing. The shooter knew what his purpose was: first to defend Christianity from Jewish influence, then to defend his European heritage and finally to make a personal sacrifice on behalf of all white Christians. He even describes his task as a sacrifice. He is prepared to die, but believes he will survive, serve his time and then return to society to complete his task.

At Poway, the threat of Christianity has joined the lethal theological threat of the Islamist terrorists. It now needs the full and unequivocal denunciation of Christian leaders across the United States and across the world.

Stephen D. Smith is Finci-Viterbi executive director of the USC Shoah Foundation.