Religion causes most of history's violence.
The objection, in full
Crusades, Inquisition, witch trials, sectarian wars, 9/11, the Troubles. Religion poisons everything by giving people license to kill in the name of certainty. A world without religion would be measurably less violent.
Religion poisons everything.
or, in plain terms —More people have been killed in the name of God than anything else.
Check the body counts.
The 20th century, the most explicitly secular century in history, killed more people for ideological reasons than all previous centuries combined. The Soviet Union, Maoist China, the Khmer Rouge, North Korea — atheist regimes, by their own self-description.
The argument needs the form: "X causes Y." But the comparison class for X (with-religion) covers most of human history; the comparison class for not-X (officially without-religion) is small, recent, and bloody.
Tribalism, scarcity, and power-seeking cause violence. Religion can recruit them. So can nationalism, ethnicity, ideology, soccer.