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.
The City of God is not the city of any flag.
I have watched Rome fall and the Christians blamed for it. I have also watched Christians who deserved blame. Both are part of one story: the visible church is a mixed body, wheat and tares together, and will be until the harvest.
The sword used in Christ's name has always been used against Christ's name. The Crusader who slaughtered the Jew of Mainz did not become more obedient to the Sermon on the Mount. He became less.
Love, and do what you will. But mark: love. Not zeal. Not certainty. Love.