One of his most significant steps up came in 1981 when he took over as head of the Congregation for the Doctrine of Faith, the Vatican office that oversees "the doctrine on the faith and morals throughout the Catholic world," according to the Vatican.
Ratzinger became known as "Cardinal No" stemming from his efforts to crack down on the liberation theology movement, religious pluralism, challenges to traditional teachings on issues such as homosexuality, and calls to ordain women as priests.
Liberation theology combined Christian theology with political activism on issues like human rights and social justice. While partially compatible with Catholic social teachings, it was rejected by the Vatican, which objected to the mixing of church theology with Marxist ideas such as class struggle.
"It (was) his job to police the doctrinal boundaries of the Roman Catholic Church, and inevitably when you do that, there are going to be hurt feelings by people who find themselves on the wrong side of those lines," said Allen, the author of "Cardinal Ratzinger: The Vatican's Enforcer of the Faith."
Over subsequent years, he made news when, for instance, he labeled homosexuality "an intrinsic moral evil" and called the Soviet Union and its communist satellite nations "a shame of our time." In the 1990s, Ratzinger openly challenged a fellow German cardinal who had encouraged divorced and civilly remarried Catholics to return to the sacraments.
Ratzinger also argued Muslim Turkey did not belong in Christian Europe and issued a document saying that Catholicism was the only true religion -- questioning the validity of other religions, even Christian ones, even as his Pope John Paul II was trying to reach out to other faiths.
Although objections came from some of his fellow cardinals, the pope did not restrain Ratzinger, in part because their friendship went back four decades, to the time when the two were young priests at the Vatican II meetings in Rome.
That bond persevered until John Paul's death in 2005, by which time Ratzinger was dean of the College of Cardinals -- the high-level group that advises the pope and, when called upon, picks a new pontiff.
In April 2005, they picked him, then 78 years old. Now called Benedict, his reputation was seen as more subdued than his predecessor. He traveled extensively, if not at John Paul's pace.
His words carried special weight, as in 2006, when he quoted from 14th-century Byzantine emperor Manuel II Paleologus regarding Islam and the Prophet Mohammed. The remarks set off protests by Muslims worldwide, and Benedict apologized for any offense taken by Muslims.
For some, the defining element of his papacy was the sex abuse scandal, with the church being accused of not doing enough to deal with reports of abuse by priests over decades, and on multiple continents. He issued a statement in 2010 saying the church had not been vigilant or fast enough in responding to the problem -- one of the most forthright admissions to date, though it still did not placate some who criticized then-Cardinal Ratzinger for not cracking down sufficiently.
Such missives demonstrated his power, in the church and on the world stage. But for all the headlines he generated, Benedict -- as he did in his first blessing after being elected pope -- insisted he was merely "a simple, humble worker in God's vineyard."
"I am consoled by the fact that the Lord knows how to work and how to act, even with insufficient tools," he said then, "and I especially trust in your prayers."