Ratzinger arrived at the papacy already carrying twenty-four years as prefect of the Congregation for the Doctrine of the Faith, the office that assesses whether a theological position is compatible with Catholic teaching. In that role he issued the 2000 declaration Dominus Iesus on the uniqueness of Christ, disciplined liberation theologians in Latin America, and became the figure critics called the enforcer and admirers called the church's intellect. His first encyclical as pope, Deus Caritas Est of 2005, surprised many by taking love rather than doctrine as its subject, and was followed by Spe Salvi on hope and Caritas in Veritate on economic justice.
The liturgical decision that defined his pontificate for many Catholics came in 2007. The motu proprio Summorum Pontificum declared that the Roman Missal of 1962, the form used before the reforms that followed the Second Vatican Council, had never been abrogated, and allowed priests to celebrate it without seeking a bishop's permission. Supporters read the ruling as a recovery of continuity, opponents as a partial reversal of the council, and the argument outlived his reign, since Francis substantially restricted the permission again in 2021.
The single most contested moment of the pontificate was academic in origin. Lecturing at the University of Regensburg in September 2006 on faith and reason, he quoted a fourteenth century Byzantine emperor's harsh remark about what Muhammad had brought to the world, intending it as a foil for an argument about violence and rationality. The quotation travelled without its frame, protests followed across several countries, and Benedict expressed regret at the reaction while declining to disown the lecture's argument.