Most people are unaware that 63 non-Catholics attended the second session of the Second Vatican Council, representatives from the Russian Orthodox, Egyptian Copts, Ethiopian Orthodox, Syrian Orthodox, the Orthodox Syrian Church of the East (India), the Apostolic Armenian Church, the Old Catholic Church, the Mar Thoma Syrian Church of Malabar, the Anglican Communion, Lutheran World Federation, World Presbyterian Alliance, the Evangelical Lutheran Church of Germany, World Methodist Council, the International Congregational Council, Friends World Committee, the World Convention of Churches of Christ (Disciples), the International Association for Liberal Christianity, Church of South India, the World Council of Churches, along with additional guests of the Secretariat for Promoting Christian Unity. Catholic critics of the Council are quick to accuse these non-Catholic groups of influencing the Council into being vague, liberal, and "modernist", however that doesn't seem to be true. Their presence at a Catholic council is an interesting milestone in ecumenical dialog that hasn't always been easy.
Leaving out the centuries of mistrust, anger, and fear that divided Christianity has caused itself, let's instead look back to the First Vatican Council's invitation to non-Catholics. The First Vatican Council was called in 1868 by Pope Pius IX. It opened in December 1869 and would be suspended in October 1870 when Italian troops invaded Rome and annexed the city (which was independent at the time). Pius IX issued a papal bull, suspending the Council indefinitely. Vatican I would never be concluded and would officially be considered closed in 1960 during the preparation of the Second Vatican Council.
Prior to Vatican I, the same desire for Christian unity was present; however, the manner in which it was expressed was a failure. In Pius IX's letter Iam vos omnes, addressed to "Protestants and other non-Catholics", Pius IX urged other Christians to "make use of the occasion of the Council" in order to "dissipate the haze of errors". In sincere hope, Pius IX urged separated Christians "to strive to free themselves from that state in which they cannot be certain about their own salvation." Needless to say, nobody showed up!
However, by the time the Second Vatican Council was announced, the Holy Spirit was drawing separated Christians closer together through common ecumenical interests, such as Scripture study and biblical archeology. Christians from around the world actively pursued the idea of unity, at least in action, through the creation of the World Council of Churches. The threats of neo-paganism via the Nazis and state-enforced atheism by the Communists found Christians from all backgrounds uniting with one another in defense of religious liberty. By the time John XXIII called for the Second Vatican Council, he announced, "a renewed invitation to the faithful of the separated churches to follow us in friendship in this search for unity and grace, desired by so many souls in all parts of the world." He shortly thereafter created the Secretariat for Promoting Christian Unity (which would one day be turned into a permanent pontifical council). Unlike in the uncharitable mindset that prevailed prior to the First Vatican Council, by this point in time there was a feeling of humility and charity toward separated Christians. Augustin Cardinal Bea, leading the Secretariat, was amazed by the attitude of the non-Catholic observers; he said it was a true miracle that so many non-Catholic Christian churches had asked their members to pray for the Council.
How much respect did the non-Catholics get during the Second Vatican Council? According to the testimony of a guest of the Secretariat, the non-Catholic observers had received all the Council texts, were able to attend all General Congregations, could make their views known at weekly meetings with the Secretariat, and they actually had personal contact with Council Fathers, the periti, and other leaders in Rome.
Despite how amazing this all is, there are many Catholics who disagree with - even despise - the reforms of the Second Vatican Council and they blame the non-Catholic presence for what they see as vague wording of Council documents so as not to offend others. However, most Catholics reject these detractors as irrational people having a bad reaction to reform. Sadly, many of them take on the attitude of Pius IX, an attitude that was natural for that day and age, but that will not work today. Browsing a "traditional Catholic" Facebook page, I saw that during the Week of Prayer for Christian Unity, they did so with wording straight out of the 19th Century in saying, "Lord, that the Anglicans will submit in obedience to the bishop of Rome, we pray for Christian unity." And each day they would replace the non-Catholic group with a new name. Smooth.
Over time the manner in which we dialog changes as attitudes change. In our day and age, charity and heartfelt discussion are what's needed in the ecumenical journey. Through dialog, we've been able to make some progress, such as the Lutherans, Catholics, and Methodists agreeing on justification or that the Reformed Christians in America now recognize the validity of Catholic baptism. There is a lot of ground to cover in order to fulfill the will of God by uniting as one Body, but if we drop the attitude and realize that it's not the 1860s anymore, we can go into this conversation knowing that there is indeed more that unites us than divides us.
For more about the workings of the Second Vatican Council, consider the book The Rhine Flows Into the Tiber.