Pope Benedict XVI, who announced his resignation Monday after eight years as head of the Roman Catholic Church, will leave a legacy of strong theology, cooperation with evangelicals and a hardline conservative stance on social issues, evangelical leaders and observers say.

As pope, the German Joseph Ratzinger championed a "culture of life" on issues such as abortion, encouraged the "new evangelism" of the church, and issued the first papal tweet. Benedict, 85, cited age and deteriorating strength as reasons for his resignation.

Benedict stepped into a difficult role as an intellectual and administrator following John Paul II, the first "celebrity pope," said Carl Trueman, professor of church history at Westminster Theological Seminary.

"He was never going to be comfortable with the kind of media exposure his predecessor courted and relished," Trueman said. "Add to that the various pressing problems facing the Roman Catholic Church — the priest abuse scandal, the challenges of feminism, Islam, rising secularism — and he took the reins at a very trying time."

In some ways, Benedict's resignation, the first in nearly 600 years, sends the most important message of his papacy, said Chris Castaldo, a director at the Billy Graham Center and author of Holy Ground: Walking with Jesus as a Former Catholic. Vatican I emphasized the divine character of the papal office and made it seem distant from average people. This resignation highlights the leader's humanity, Castaldo said.

"He's expressing vulnerability in such a way that others will be able to identify with," he said. "There's a temptation for all of us — Protestants, Catholics — in leadership with the church in clerical positions to give the impression that we have it all together, that we're completely up for the challenge and the task. There are many times in life when we're not."

During the past eight years, Benedict aided evangelicals with his strong support for traditional Catholic stances on abortion, the sanctity of life and traditional marriage, said Leith Anderson, president of the National Association of Evangelicals. Anderson, who met Benedict at an ecumenical prayer service in Manhattan in 2008, said he thanked Benedict at that gathering for his strong pro-life stance on everything from abortion to end-of-life topics.

"As evangelicals, those are really important concerns for us. Often, if it weren't for the Roman Catholic Church, we'd be standing alone," Anderson said. "I'm grateful and impressed with his faithfulness in these areas."

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Benedict's ecumenism also is evident in the document "Christian Witness in a Multi-Religious World," a several-page booklet that the Catholic Church, the World Council of Churches and the World Evangelical Alliance coauthored in 2010. The document lists biblical support for Christian witness and gives principles and recommendations for inter-faith engagement. United Nations officials and governments are also using the pamphlet to work with faith-based initiatives, said Geoff Tunnicliffe, secretary general of the World Evangelical Alliance.

Across the board, Benedict followed the tradition of Pope John Paul II, who held office from 1978 to 2005 and is known for holding to traditional, socially conservative policies within the Catholic Church, said Russell Moore, dean of the school of theology at Southern Baptist Theological Seminary. That conservatism frustrated many who wanted to see the church move toward acceptance of same-sex marriage.

"His legacy is one of speaking forcefully in defense of old truths," Moore said. "In an era that defined progress as capitulation to the sexual revolution, Benedict stood firm for the permanent things of human dignity and life and marriage."

The pope's tenure was troubled by some missteps, said Richard Mouw, president of Fuller Theological Seminary. Benedict began a major project to reform women's religious orders in the U.S. while the sex abuse scandal was ongoing in the church, which was unhelpful, Mouw said. The pope also reinforced traditional Catholic teachings on Mary and papal infallibility, emphasizing the theological divide between Protestants and Catholics.

"Many of us — especially since the reforms of Vatican II — have come to see the pope as one of the important teachers in the Christian world," Mouw said. "As a teacher, I would give Benedict a 'B.'"

In general, though, Benedict emphasized ecumenism in much of his writing, said Francis Beckwith, a philosophy professor at Baylor University who returned to Catholicism while serving as president of the Evangelical Theological Society in 2007. Many of Benedict's encyclicals and other works are about faith, hope and love, Beckwith said.

"To emphasize these aspects of the gospel does have a great ecumenical pull to it," he said. "If you look at the way in which his writings have come out of the Vatican, underneath it all is a desire for Christian unity."

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Timothy George, dean of Beeson Divinity School, has worked with Benedict on ecumenical coalitions, most recently to discuss the importance of evangelism. At that meeting last fall, Benedict emphasized the importance of Bible translation to make it available around the world and the importance of Christians working together to spread the gospel.

"The pope realizes that Christians need to stand together, speak out together and be clear about the gospel together in the world in which we live today," George said.

Benedict was the first theologian to take on the papal role in the last century, George said. The pope has written on the doctrine of the church and ecumenism and recently finished a trilogy on the life of Jesus.

"I can't think of anybody else who brought to the papacy the kind of theological acumen, savvy, and depth that Benedict XVI did," George said. "That will be a continuing legacy, to reaffirm the historic theological position of the Catholic Church."

Editor's note: An earlier version of the story characterized the First Vatican Council as emphasizing the "divinity of the pope." We meant to say that it "emphasized the divine character of the papal office." The Catholic Church has never taught that the pope is himself divine. The communication error was ours, not our source's.