One was a high-profile theologian from Switzerland, a towering figure of 20th-century Christianity whose writings brought him a global following, but who could be spiky and fiercely critical of Rome. The other was a mild-mannered, affable Australian, a skilled Vatican diplomat who largely worked behind the scenes as he rose to one of the highest positions in the Roman Curia.

Father Hans Küng and Cardinal Edward Cassidy died within a few days of each other (Küng on April 6, Cassidy April 10) and, at first glance, appear to share little in common.

Yet Küng and Cassidy share an important legacy when it comes to Christian unity, even though they pursued this goal in very different ways. Their lives offer lessons in how reform and renewal in the church is brought about by those who choose to work from inside the central institution and those who prefer to keep a critical distance. 

Let’s start with Küng. His doctoral thesis on the doctrine of justification, where he boldly argued that the old theological dividing line between Protestants and Catholics at the Reformation could now be overcome, was published in English in 1964. Küng made the case that the differences between denominations over justification — the question of how God, through Christ, removes the penalty of sin and opens the way to salvation — were only “imaginary.” His provocative thesis led the doctrine office in Rome to open a file on him.

Now Cardinal Cassidy. Thirty-five years later, the Australian cardinal was president of the Holy See’s Pontifical Council for Promoting Unity, when he signed a groundbreaking Catholic-Lutheran agreement on the Doctrine of Justification. This 1999 text was hailed as a major ecumenical achievement, effectively resolving the long-running theological dispute.

While Küng’s theological research had laid some of the groundwork for this agreement, it was the patient diplomacy of the Australian cardinal that secured the agreement. The fruits of their labors made it possible for Pope Francis to travel to Sweden in 2016 to mark 500 years since the Reformation and declare: “The doctrine of justification thus expresses the essence of human existence before God.”

Cardinal Cassidy applied his diplomatic skills to ecumenical relations. For decades, he served in Holy See missions across the world, including in El Salvador, where he became friends with Archbishop Oscar Romero, and in South Africa during apartheid, where he negotiated the appointment of the first non-white bishops.

He would later be appointed “sostituto” at the Secretary of State, the equivalent of papal chief of staff. He held this position for just 18 months before he was given the brief to work on Christian unity and the church’s relationship with Jews. Although he felt himself unqualified, it was a role he embraced with enthusiasm.

The stories of Küng and Cardinal Cassidy show how the church needs both the prophetic work of the theologian — to think, and argue for what might seem unthinkable — and the patient, skillful diplomacy of relationship building that can advance institutional reforms when the time is right.

Küng liked to move quickly. French theologian Yves Congar, one of the architects of the Second Vatican Council, in which Küng also played an influential part as a theological adviser, described his colleague as going “straight ahead like an arrow.” He added that he was a “demanding, revolutionary type, rather impatient.” Küng liked to throw bricks over the Vatican’s walls, but in 1979, after questioning the doctrine of papal infallibility, Rome’s doctrinal office revoked his license to teach Catholic theology. 

Nevertheless, he remained priest in good standing until his death at 93, and made his arguments out of a “loyal commitment” to the church. He unnerved people in the Vatican precisely because of his understanding of the Roman system and his ability to critique it.

Yet Küng wanted a church that could more credibly witness to the saving message of Jesus Christ. The theologian’s work in intra-church and interreligous dialogue will be his enduring legacy. The latter is central to the ministry of this pope and the Holy See, and Küng felt reconciled to Rome during the Francis papacy.

Cardinal Cassidy, meanwhile, liked to work toward ecumenic goals more gradually.

“When I waited for him in a meeting room, you could hear him coming with his very specific, very fast, small steps. And that was also his approach to ecumenism: you need (a lot of) small steps,” as Father Oliver Lahl, a German ecumenist who worked with Cassidy’s successor Cardinal Walter Kasper, told me.

“He was able to compromise and to understand, that to widen the common ground, both sides had to move.”

When faced with strong disagreement and bold theological ideas, the temptation for some in Rome is to rush rulings to end the discussion. The model of patient dialogue practiced by Cardinal Cassidy offers the lesson, however, that on contentious issues Rome should give time and space for discernment. Disagreement, debate and even dissent should not be viewed as a threat but as part of the journey to reaching a judgment. The likes of Küng can be demanding and difficult. But, as Congar concluded, “we need such people.”