The Crusades and the Inquisition show that when religion gains power, it produces violence and persecution — proof that Christianity is a dangerous force, not a moral good.

Why people find it persuasive

These are well-documented historical episodes involving real violence carried out explicitly in the name of Christian faith and under church authority, sanctioned or directed by popes and religious courts, which seems like straightforward historical evidence that religious conviction, especially when politically empowered, tends toward violence.

Important distinctions

• • Religiously-motivated violence vs. politically or economically motivated violence carried out under religious justification or branding.
• • The actual, historically documented scale and character of these events vs. inflated popular death-toll figures and caricatures.
• • A general claim about 'religion' as a category vs. specific historical claims about particular medieval institutions in a particular political context.
• • Whether these actions were consistent or in tension with the actual moral teaching of the New Testament.

Christian response

• • Engage the actual history rather than either minimizing or exaggerating it: modern historians (like Jonathan Riley-Smith on the Crusades and Henry Kamen on the Spanish Inquisition) have significantly revised popular death-toll and motivation narratives — the Inquisition's execution numbers, for instance, are documented in surviving trial records to be far lower than centuries of Protestant and Enlightenment-era polemical propaganda claimed, without this making the actual persecution and executions that did happen acceptable.
• • Point out the Crusades emerged from a genuinely complex mix of motives — a defensive response (however badly and violently executed) to centuries of prior territorial conquest and hostility, feudal politics, economic incentive, and religious fervor intertwined together — historians broadly agree that reducing them to pure 'religion causes violence' oversimplifies a event with substantial political and military-historical causation running alongside the religious framing.
• • Bring in comparative history: the twentieth century's explicitly atheistic and secular totalitarian regimes (Stalin's USSR, Mao's China, Pol Pot's Cambodia) produced death tolls vastly exceeding anything attributable to religious violence across all of history combined, which shows that removing religion does not eliminate large-scale ideologically-motivated violence, and undercuts the inference that religion as such is uniquely or specially dangerous compared to secular ideology wielding comparable power.
• • Distinguish the moral content of Jesus' own teaching (love of enemies, non-retaliation, the Sermon on the Mount, rejection of Peter's use of the sword in Gethsemane) from the conduct of later medieval institutions that, whatever their professed faith, plainly and directly violated those teachings — historically, this tension was noted at the time by dissenting Christian voices as well, showing it was recognized within the tradition, not only by later outside critics.
• • Note that using historical evils to argue against present-day Christian claims requires the same move critics rightly resist elsewhere: judging a set of ideas entirely by the worst historical conduct of some of its adherents rather than by their actual content and by the full historical record, including extensive positive contributions (hospitals, universities, abolitionist movements, and relief and human rights work substantially motivated by Christian conviction) that a fair historical accounting must also weigh.

Christian counterresponse

Historians of religion and violence (like Karen Armstrong, from a more sympathetic angle, or William Cavanaugh in 'The Myth of Religious Violence') argue this 'religion is uniquely and structurally more dangerous' claim does not hold up well comparatively once you examine how thoroughly non-religious ideologies (nationalism, communism) have also treated disagreement as absolute and non-negotiable and have produced comparable or greater violence when fused with state power — suggesting the danger lies in the fusion of any totalizing ideology with unaccountable political power, not in religious content specifically.