Paolo Gabriele, 46, has admitted taking confidential documents and leaking them to the Italian media – although no guilty plea has been entered.
He has told investigators that he was hoping to expose “evil and corruption” within the Church.
While technically he faces up to four years in prison if found guilty, Paolo Gabriele could be pardoned by the Pope.
If he is jailed, he will serve his sentence in an Italian prison as Vatican City has no long-term detention facilities on its territory.
Paolo Gabriele is standing trial along with Vatican computer technician Claudio Sciarpelletti, who is accused of aiding and abetting a crime.
He was the Pope’s trusted servant for years and held the keys to the papal apartments.
Many of the letters and other documents he took from the pontiff’s desk were published in a book by an Italian investigative journalist in May.
The so-called “Vatileaks” scandal has sparked allegations of corruption and internal conflicts at the Holy See.
It has been one of the most difficult crises of Pope Benedict’s seven-year papacy.
No TV cameras or recorders are being allowed inside the courtroom for the most high-profile case to be held in the Vatican since the Holy See was established as a sovereign state in 1929.
The Vatican was arrested in May, accused of passing papal correspondence to journalist Gianluigi Nuzzi, whose book His Holiness: The secret papers of Pope Benedict XVI was published that month.
Some of the most sensational letters were written to the Pope by Archbishop Carlo Maria Vigano, currently the Vatican’s ambassador to Washington, who was deputy governor of Vatican City at the time.
In one letter, Archbishop Vigano complains that when he took office in 2009, he discovered corruption, nepotism and cronyism linked to the awarding of contracts to outside companies at inflated prices.
He later writes about a smear campaign against him by other Vatican officials upset at his actions to clean up purchasing procedures.
The archbishop begs in vain not to be moved away from the Vatican as a punishment for exposing the alleged corruption.
Correspondents say the revelations seem aimed primarily at discrediting the Vatican’s powerful Secretary of State, Cardinal Tarcisio Bertone, who has been in his post since 2006.
Prosecutors quoted Paolo Gabriele as saying during his interrogation that he knew taking the documents was wrong but he felt the Holy Spirit was inspiring him to shed light on the problems he saw around him.
He said he felt the Pope was being kept in the dark or misinformed by his collaborators.
“Seeing evil and corruption everywhere in the Church… I was sure that a shock, even a media one, would have been healthy to bring the church back on the right track,” he was quoted as saying in June.
Pope Benedict said after his former butler’s arrest that the news had “brought sadness in my heart”.
Psychologists were summoned by the Vatican to determine whether Paolo Gabriele could be held responsible for his actions.
The results were conflicting.
One report concluded that while he could be held accountable for his actions, he was socially dangerous, easily influenced and could “commit acts that could endanger himself or others”.
This report described Paolo Gabriele as subject to ideas of “grandiosity”, as attention-seeking and as a simple man with a “fragile personality with paranoid tendencies covering profound personal insecurity”.
Another report cited in the indictment concluded that the defendant, a 46-year-old father of three, had shown no signs of major psychological disorder or of being dangerous.
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