As Cardinal Ratzinger, he was for 25 years the head of the Congregation of the Doctrine of the Church (once the Holy Office responsible for the Inquisition), which was the office charged with defending Church orthodoxy. When Pope John Paul II (now Saint) died in 2005, Ratzinger was chosen to become the new Pope. He stunned the world in 2013, when he resigned the Papacy, because that had not been done in almost 700 years. He continued to live as "emeritus Pope" living on the grounds of the Vatican, which seemed to challenge the authority of Pope Francis, particularly since Benedict was a favorite of the conservative Catholics as opposed to the liberal Catholics who favored Francis.
Benedict's position in the Church was complicated. In condemning relativism and secularism and defending the traditional orthodoxy of the Church, he seemed to be a conservative. But he was also a theistic evolutionist, a homosexual who condemned homosexuality, and a Lockean liberal in asking forgiveness for the Church's history of religious violence and in promoting religious toleration.
Over the years, I have written a series of posts on Benedict's moral and theological ambiguity as a manifestation of the intellectual crisis of the Catholic Church.