Vatican II, convoked by Pope John XXIII, was the first truly global council of the Church, bringing together bishops from 116 countries. It opened the windows of the Church to the world - but some have found the resulting breezes unsettling.
The newly elected Pope John XXIII stunned the world in 1959 by calling the first Church Council in nearly a century — the Second Vatican Council, or what's known as Vatican II.
He called for the institution's renewal and more interaction with the modern world.
As a result of Vatican II, the Church opened its windows onto the modern world, updated the liturgy, gave a larger role to laypeople, introduced the concept of religious freedom and started a dialogue with other religions.
But the changes provoked a backlash.
Over a three-year period, more than 2,000 bishops from all over the world, assisted by thousands of advisers, issued 16 landmark documents.
No new dogma was issued, but the council transformed the Church from an exclusive to an inclusive institution. Before the Second Vatican Council, altars were turned so the priests celebrated Mass with their backs facing the congregation.
Vatican II decreed that altars should be turned around, and priests faced the newly recognised people of God — that is, the entire community of Catholic believers, not just the clergy or church hierarchy.
The Council allowed priests to celebrate Mass in the local language, thus making a key sacrament more accessible in the contemporary world.
At the time of Vatican II, the Rev. Thomas Reese was studying at a seminary so isolated from the world that students were unaware the council was taking place. But its effects, he says, were abrupt.
'One week, if you eat meat on Friday, you're going to go to hell. The following week, you can have meat on Friday,' he says. 'The Church changed.'
Not all issues could be discussed at the Council - for example, priestly celibacy and the role of women remained off-limits.
However, Vatican II gave bishops a sense of empowerment. They spoke candidly, and many openly criticised the Vatican ban on artificial birth control.
Robert Mickens, Vatican correspondent for the British Catholic weekly The Tablet says the bishops' assertiveness shocked the hierarchy.
'This caused much concern among the conservatives, that this was undermining unity in the Church, undermining the power and the voice of the Pope,' he says, 'so slowly there was a clawing back.'
Pope John's successors, Pope John Paul II, who was from Poland, and Pope Benedict XVI, of Germany, were both present at the Council and were among those who felt its effects went too far.
As Pope, John Paul, with then-Cardinal Joseph Ratzinger — now Pope Emeritus Benedict — by his side, introduced what's known as the restoration and appointed bishops loyal to the Vatican. The major casualty of this restoration was collegiality, the concept that bishops had a role in the decision-making process.
Read full article: Vatican II: A Half-Century Later, A Mixed Legacy (NPR)
Mgr Kevin Barry-Cotter: Reflections on the graces of Vatican II (Archdiocese of Canberra Goulburn)
'The biggest meeting in the history of the world' (The Tablet)
Here to stay: Women and the Council (The Tablet)
Bishop Comensoli: How to Reform the Church: Learning the Lessons of Vatican II (ABC Religion and Ethics)
Documents of the Second Vatican Council (Vatican)