ROME — Marking the 70th birthday of German Cardinal Gerhard Ludwig Müller, emeritus Pope Benedict XVI said that even though the cardinal is no longer prefect of the Congregation for the Doctrine of the Faith, he will continue to have a public role of serving the church.

The retired pope wrote the introduction to a book of essays honoring Müller on his 70th birthday Dec. 31 and in anticipation of the 40th anniversary of his priestly ordination in February.

Released in early December by the German publisher Herder, the book is titled, The Triune God: Christian Faith in the Secular Age.

In the introduction, Benedict wrote that Blessed Paul VI wanted the more demanding positions in the Roman Curia, particularly the posts of prefect and secretary of the congregations, “to be limited to only five years in order to protect the freedom of the pope and the flexibility of the Curia.”

Addressing Müller, Benedict said, “your five-year commission at the Congregation for the Doctrine of the Faith has expired, so you do not have a specific office anymore, but a priest and certainly a bishop and cardinal is never simply retired,” which is why he must continue to “publicly serve the faith.”

Vatican Radio said Benedict also spoke about the importance of being prefect of the doctrinal congregation — a position he held for more than 23 years during the pontificate of St. John Paul II. Theological knowledge is essential for the role, the retired pope said, but so is knowing the limits of one’s theological knowledge.

Benedict concluded his introduction by telling Müller, “You have defended the clear traditions of the faith, but in the spirit of Pope Francis you have tried to understand how they can be lived today.”

When it was announced in July that Müller’s term was not being renewed, many bloggers and opinion writers presented Francis’s decision as one of firing the German cardinal because of theological disagreements, particularly over the issue of Communion for divorced and civilly remarried Catholics.

Müller responded by telling the German daily, Allgemeine Zeitung, that “there were no disagreements between Pope Francis and me.”

The cardinal told the newspaper that the pope’s decision had been unexpected since such terms were usually renewed, but that he was not bothered by it.

As a cardinal under the age of 80, Müller still is a member of a number of Vatican congregations and councils, so “I have plenty to do in Rome.”