Israel’s true enemy is not external. Baseless hatred, known in Hebrew as Sinat Chinam (שנאת חינם), is far more dangerous than the hostility emanating from Iran, Hezbollah or Hamas. The recent day of mourning and fasting known as Tisha B’Av is all about remembering the extent of this particular threat.

Known as the saddest day on the Jewish calendar, Tisha B’Av commemorates the destruction of both the First and Second Temples. To mark the date, a dialogue took place in the Jewish settlement of Efrat between Bethlehem and Hebron. It featured, among others, Israeli Arab news anchor Lucy Aharish and Jewish Temple Mount activist Yehudah Glick, accused by many of being a “right-wing extremist.”

Why was the First Temple destroyed? Because of three evils that had infected it: idolatry, fornication and bloodshed. Why was the Second Temple destroyed? Because there was baseless hatred among the people. In other words, baseless hatred for others is as severe a sin in...