Gregory Baum, author of the first draft of Nostra Aetate, was 94 years old.

Baum was admitted to St. Mary’s Hospital in Montreal Oct. 8. “I’m disappearing inside,” he told a friend. He decided not to continue the dialysis treatment which had kept him alive the last four years.

As a young theologian, Baum shot to prominence in the early days of the Second Vatican Council, mentored by Cardinal Augustin Bea. A key ally of Pope St. John XXIII, Bea looked for credible Catholic experts on Catholic-Jewish relations and found his man in Baum.

Baum had been born to a Jewish mother and Protestant father in Berlin in 1923. In 1940, at the age of 17, he came to Canada as a war refugee after a brief stay in England.

He became a Catholic during the war years and joined the Augustinian order in 1947. He was ordained a priest in 1954. He studied theology at the University of Fribourg in Switzerland and published That They May Be One, an influential book about Catholic ecumenism in 1958.

His involvement in the Second Vatican Council (1962-65) began even before the world’s bishops met in Rome, as Vatican officials were planning the Church’s first truly global meeting.

“I remember the first session I attended was in November 1960,” Baum told The Catholic Register in 2012. “I was at the first session of the secretariat in Rome. We had the first meeting with Cardinal Bea and Monsignor (later Cardinal Johannes) Willebrands, and this was all about ecumenism. At the end of the meeting Cardinal Bea said, ‘I just saw the pope and he said to us, he said that he wants the secretariat to prepare a statement to rethink the Church’s relationship to the Jews.”

St. John XXIII’s concern about the six million Jews killed in the heart of Europe during the Second World War largely drove the Second Vatican Council. Baum had already begun publishing in academic journals about Catholic-Jewish relations.

Baum attended all three sessions of the Council as a peritus or theological expert, consulting on Nostra Aetate (On the Church’s Relation to Non-Christian Religions), Unitatis Redintegratio (On Ecumenism) and Dignitatis Humanae (On Religious Liberty).

After the Council, Baum became a professor of theology and ethics at the University of St. Michael’s College in the University of Toronto. He eventually left the priesthood in 1974 and married. He studied sociology at the New School for Social Theory in New York and in the 1980s moved to the Religious Studies department at McGill University in Montreal.

Baum was a frequent target of conservative campaigners in English Canada and the United States. Canonist Msgr. Vincent Foy frequently published articles condemning Baum as a “marxist... ex-priest.” Foy popularized a theory that Baum had excommunicated himself by marrying before his laicization was formally recognized by the Vatican. Baum’s opinions in favour of ordaining women and gay marriage came in for frequent criticism.

Baum’s critics were further incensed by his 2016 autobiography The Oil Has Not Run Dry, in which he spoke of his first homosexual experience at the age of 40.

The author of more than 20 books, Baum said he was never worried by the criticism.

“I live in a dream world in Quebec,” he told The Catholic Register. “I still belong to a wide network of progressive Catholics. I never meet any conservatives.”

In Montreal, Baum is mourned as a great friend of Quebec.

“Anyone who wants to understand his theological thinking must see Gregory’s human qualities. He had the charisma of friendship, a faithful friendship,” Montreal theologian Michel Beaudin told the Quebec religious news service Presence. “Gregory came to us, he adopted Quebec, he married the destiny of Quebec. In his outward-looking writing, he valued what Quebec is.”

In Toronto theologians and friends will gather for a memorial Mass at Regis College’s St. Joseph Chapel at 2 p.m. Nov. 5.

“We have lost a great-hearted friend, a magical smile, a wise and funny conversationalist, a quietly serious lover of the Church and a less quiet but totally passionate theologian who worked beyond fatigue to articulate, with the heart of his very living community, the intersection of human longing and God’s loving of the world,” Jesuit Fr. Jack Costello wrote in an emailed notice for the memorial Mass.

Baum’s books reached far beyond academic, theological circles, said Novalis English publishing director Joe Sinasac.

“He continued to work indefatigably to help Catholics the world over to understand that being Catholic meant much more than obeying the rules and going to Church on Sunday,” Sinasac wrote in an email. “He truly encapsulated the spirit of Vatican II.”