As his stances hardened, the nicknames piled on from "God's Rottweiler," to "the German Shepherd" to "Cardinal No."
As John Paul II's health declined, Ratzinger took over more and more responsibility at the Vatican. In 2002, he became dean of the College of Cardinals, which didn't seem to slow down his prolific writing.
The pontiff authored more than 30 titles including: "Pilgrim Fellowship of Faith: The Church as Communion," "Truth and Tolerance: Christian Belief and World Religion," "The Spirit of the Liturgy" and "Milestones: Memoirs 1927-1977."
In the days after Pope John Paul II's death April 2, 2005, the shy and humble Ratzinger stepped into the spotlight. He delivered a heartfelt homily at John Paul's funeral followed by a fiery speech to the cardinals.
Before the cardinals started the secret process of choosing a successor, he warned them about tendencies that he considered dangers to the faith: sects, ideologies like Marxism, liberalism, atheism, agnosticism and relativism -- the ideology that there are no absolute truths.
"Having a clear faith, based on the creed of the church is often labeled today as a fundamentalism," he said. "Whereas relativism, which is letting oneself be tossed and swept along by every wind of teaching, looks like the only attitude acceptable to today's standards."
Despite thoughts that his conservative penchant would keep him out of the running, Ratzinger's election to St. Peter's throne was swift.
His election in four ballots in two days was one of the shortest in 100 years. He is also the oldest pope elected since Clement XII in 1730.
If Ratzinger was paying tribute to the last pontiff named Benedict, it could be interpreted as a bid to soften his image as "God's Rottweiler." Benedict XV reigned during World War I and was credited with settling animosity between traditionalists and modernists, and dreamed of reunion with Orthodox Christians.
The name he took draws a connection to Benedict XV, the Italian pontiff from 1914 to 1922 who had the difficult task of providing leadership for Catholic countries on opposite sides of World War I. His declared neutrality and humanitarianism was demonstrated in his untiring efforts to relieve the sufferings of the war.
He founded a bureau for the exchange of wounded prisoners and a missing-persons bureau and established relief agencies.
Benedict was also known for his outreach to Muslims and efforts to close the nearly 1,000-year estrangement with Christian Orthodox churches.
In his homily after assuming the role of pope, Pope Benedict XVI told the faithful, "The Church is young." As a cardinal known for his strict orthodoxy, Benedict struck a softer note. He said he did not need to lay out a governing program.
Immediately after the Mass, a white Jeep pulled up to the steps of the basilica, and Pope Benedict XVI climbed in. There was no bulletproof glass as he drove around for what was described as a victory lap.
In his first weeks as Pope, Benedict pledged to work for reconciliation and peace among peoples.
He also referred to the Christian roots of Europe, in what was a major theme of his papacy.
In his only visit to the United States as pope, Benedict was welcomed for an elaborate state visit arrival at the White House in 2008. President Obama was granted an audience at the Vatican just months after taking office, although the Obama administration and the church have clashed on important issues, including abortion, contraceptive services, and stem cell research.