President Biden visited the Apostolic Nunciature to the United States in Washington, D.C., on Thursday evening to sign the book of condolences for the passing of Pope Benedict XVI.

Biden made the unannounced trip to pay respects after the conclusion of the late pontiff's funeral in Rome the same day. The president was not invited due to Benedict's request for "simplicity."

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President Biden signs Pope Emeritus Benedict XVI condolence book

President Biden, left, signs the condolence book for Pope Emeritus Benedict XVI with Archbishop Christophe Pierre at the Apostolic Nunciature of the Holy See in Washington, D.C., Thursday, Jan. 5, 2023. (Sarah Silbiger/Bloomberg via Getty Images)

An apostolic nunciature is similar to an embassy, but instead represents the Bishop of Rome's interests abroad. 

Biden met briefly with Apostolic Nuncio Archbishop Christophe Pierre, calling the opportunity to visit an honor.

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A photo of the coffin and people at the funeral

The coffin of late Pope Emeritus Benedict XVI is brought to St. Peter's Square for a funeral mass at the Vatican, Thursday, Jan. 5, 2023 (AP Photo/Ben Curtis)

The president spoke to reporters Wednesday outside the White House and was asked for his thoughts on the pope emeritus's passing.

"He was a great, and it reminded me of going back to theology class. We spoke about Aquinas and about ‘Summa Theologica’ and the whole litany," Biden said of Benedict, a pope who had served at the highest levels of academia. "I found him to be relaxing and very rational."

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President Biden arrives at Holy See embassy

President Biden, right, speaks with Archbishop Christophe Pierre before signing the condolence book for Pope Emeritus Benedict XVI at the Apostolic Nunciature of the Holy See in Washington, D.C., Thursday, Jan. 5, 2023. (Sarah Silbiger/Bloomberg via Getty Images)

The "Summa Theologica" or "Summary of Theology" is a massively popular theological text written by St. Thomas Aquinas in the 13th century. 

"He was a more conservative view within the Catholic realm than I have — I'm much closer to pope, the present pope, in terms of his philosophy, his view. But I admired him. I thought he was a fine man," Biden added.

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President Biden signs Pope Emeritus Benedict XVI condolence book

President Biden signs the condolence book for Pope Emeritus Benedict XVI at the Apostolic Nunciature of the Holy See in Washington, D.C., Thursday, Jan. 5, 2023. (Sarah Silbiger/Bloomberg via Getty Images)

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On not attending the funeral, Biden said, "The reason I’m not attending the funeral tomorrow is because it takes an entourage of a thousand people to show up. Not literally, but we would move everything in the wrong direction."