When Pope John XXIII announced in January 1959 that he intended to convene an ecumenical council, many in the Roman Curia were reportedly startled — the church, after all, had only recently emerged from the era of Vatican I and seemed doctrinally settled. But John XXIII, elected as what many expected to be a transitional, low-key pope, wanted something different: an aggiornamento, an “updating,” that would help the church engage more effectively with the modern world. He famously described his goal as opening a window to let in some fresh air.

An Unprecedented Gathering

The Second Vatican Council opened on October 11, 1962, in St. Peter’s Basilica, and would ultimately involve up to 2,625 bishops from around the globe across four sessions, concluding in December 1965 under Pope Paul VI following John XXIII’s death in 1963. Unlike Trent or Vatican I, which had been convened primarily to condemn specific errors, Vatican II was framed from the outset in a more pastoral and dialogical spirit, aiming to renew the church’s own life and its relationship with the modern world rather than to define new dogma or anathematize opponents.

Sweeping Changes to Worship

The council’s most immediately visible impact was on the liturgy. The constitution Sacrosanctum Concilium called for the Mass to be celebrated in the vernacular language of local congregations rather than exclusively in Latin, encouraged greater lay participation in worship, and reoriented altars so that priests generally faced the congregation. For millions of Catholics accustomed to the Tridentine Mass, these changes represented the most dramatic shift in Catholic worship in four centuries.

Rethinking the Church’s Relationship to the World

Vatican II produced several landmark documents that reshaped Catholic theology and practice. Lumen Gentium offered a renewed vision of the church as the “People of God,” emphasizing the shared dignity and mission of all the baptized, not just clergy and religious. Dei Verbum addressed the relationship between Scripture and tradition. Gaudium et Spes engaged directly with the joys, hopes, griefs, and anxieties of the modern world, addressing topics from economic justice to war and peace.

A New Posture Toward Other Faiths

Perhaps most strikingly, the council issued Nostra Aetate, a groundbreaking declaration on the church’s relationship with non-Christian religions that repudiated antisemitism and called for greater interfaith understanding, and Unitatis Redintegratio, which committed the Catholic Church to the modern ecumenical movement and a more conciliatory posture toward other Christian traditions — a posture that, within a few years, led Pope Paul VI and Ecumenical Patriarch Athenagoras I to formally lift the mutual excommunications that had stood since the Great Schism of 1054.

An Ongoing Debate About Meaning

More than half a century later, Vatican II remains one of the most debated events in modern Catholic history. Some view it as a long-overdue and necessary renewal that allowed the church to speak meaningfully to the contemporary world; others argue that its reforms went too far, too fast, or were implemented in ways that departed from the council’s actual texts and intentions. What is not in dispute is its scale: Vatican II fundamentally reshaped the everyday experience of Catholic worship and the church’s public posture toward the modern world in ways still felt in every Catholic parish today.

The First Vatican Council opened on December 8, 1869, in St. Peter’s Basilica, convened by Pope Pius IX — the first ecumenical council to meet in three centuries, since the Council of Trent had adjourned in 1563. It would prove to be dramatically cut short by world events, yet it produced one of the most significant and controversial doctrinal definitions in the modern history of the Catholic Church: papal infallibility.

A Church Under Siege

Pius IX called the council amid a sense of crisis. The nineteenth century had brought waves of political liberalism, secularism, and nationalism that Pius viewed as direct threats to both the church’s authority and its temporal power. The Papal States, which the pope had ruled as a sovereign temporal power for over a thousand years, were under direct military threat from the movement for Italian unification. Pius hoped the council would help the church speak with unified strength against what he saw as the errors of the modern age — a concern he had already articulated forcefully in his 1864 Syllabus of Errors.

The Question of Papal Infallibility

Of the many items on the council’s agenda, none proved more contentious than the proposal to formally define papal infallibility — the doctrine that the pope, when speaking ex cathedra (“from the chair”) on matters of faith and morals binding on the whole church, is preserved from error. A significant minority of bishops, while not necessarily rejecting the underlying idea, worried that a formal definition was untimely, poorly worded, or would inflame tensions with Protestants, Orthodox Christians, and secular governments.

Despite this opposition, the council’s constitution Pastor Aeternus, approved in July 1870, formally defined papal infallibility under specific and narrowly defined conditions. It was one of the most significant assertions of centralized papal authority in the church’s history.

Cut Short by War

The council never reached a formal conclusion. Weeks after Pastor Aeternus was approved, the Franco-Prussian War broke out, and French troops that had been protecting Rome and the remaining Papal States were withdrawn. In September 1870, Italian nationalist forces captured Rome itself, ending over a millennium of papal temporal rule. Pius IX indefinitely suspended the council, and it was never formally reconvened; it remained technically open, in a state of suspension, until Pope Paul VI formally closed it decades later, ahead of the Second Vatican Council.

A Doctrine, and a Church, Transformed

The loss of the Papal States forced the papacy to redefine its role for the modern age — no longer as a temporal sovereign ruling Italian territory, but increasingly as a global spiritual authority. The doctrine of papal infallibility, meanwhile, became a defining and often contested feature of Catholic identity, shaping relations with other Christian traditions and setting the doctrinal stage that the Second Vatican Council would later engage with a century later.