(CNN) — If Paolo Gabriele ever does get the pope's pardon, it won't be before he serves some jail time.
Gabriele, a former butler to Pope Benedict XVI, will start his 18-month sentence in a Vatican cell Thursday for taking secret papers from the pope's personal apartment and leaking them to an author who included them in a best-selling book, the Vatican said.
Gabriele, one of the pope's closest personal assistants, was convicted in Vatican City court October 6 of aggravated theft. He was arrested in May, following a Vatican investigation into how the pope's private documents appeared in the book "Sua Santita" ("His Holiness") by Italian journalist Gianluigi Nuzzi. The book, based on the papers, revealed claims of corruption in the church's hierarchy -- claims that could affect who becomes the next pope.
The Holy See's media office has repeatedly said that the pope could pardon Gabriele. But on Thursday, Vatican Secretary of State Cardinal Tarisco Bertone said that Gabriele would first have to express repentance and make "a sincere request for pardon to the supreme pontiff and those who have been unjustly offended."
In court this month, Gabrielle declared himself not guilty, but said he had abused the pope's trust. His lawyer told the court that while his actions were illegal, they were morally motivated and she hoped that one day "they will be recognized and applauded."
He did not appeal his sentence, according to the Vatican.