Second Vatican Council
The Second Ecumenical Council of the Vatican, commonly known as the or , addressed relations between the Catholic Church and the modern world. The council, through the Holy See, was formally opened under the pontificate of Pope John XXIII on 11 October 1962 and was closed under Pope Paul VI on the Solemnity of the Immaculate Conception on 8 December 1965.
Catholic Church and ecumenism
"The search for Christian Unity was one of the principal concerns of the Second Vatican Council."
This was attested to even from the moment the Council was announced, by Pope John XIII, during the vespers closing the Octave of Prayer for Christian Unity, on 25 January 1959, at the Basilica of Saint Paul's Outside the Walls.