Pope Benedict XVI's former butler Paolo Gabriele takes the stand on Tuesday at his historic trial for stealing secret memos in what he said was a bid to battle "evil and corruption" in the Vatican.
It will be the first time that Gabriele speaks out since his arrest in May 2012 and his 53-day detention in a Vatican security cell.
Gabriele, who is now under house arrest, is accused of leaking hundreds of memos that reported fraud and intrigue among senior Vatican figures.
At the start of the trial on Saturday, Gabriele suffered a series of setbacks when judges turned down his lawyer's requests to strike down his indictment and throw out the case because of rules on papal secrecy.
Judges also declined to include in the trial a top secret report on the "Vatileaks" scandal compiled by a committee of cardinals appointed by the pope who interviewed dozens of people in a parallel investigation into the leaks.
The pope's secretary and closest confidant, Monsignor Georg Gaenswein, and one of the four lay housekeepers who help the 85-year-old pope go about his daily life, Cristina Cernetti, are expected to testify later in the trial.
A German like the pope, Gaenswein is a hugely influential figure at the Vatican as he is effectively the filter for the pope's interactions with the outside world and his role has sparked jealousy behind the Vatican walls.
The first hearing also revealed that Vatican gendarmes had seized 82 boxes of material from Gabriele's service apartments in the Vatican and at the pope's summer residence and installed a camera on the landing of his Vatican flat.
Gabriele gave only one interview in February to Gianluigi Nuzzi, the Italian investigative journalist to whom he is accused of having leaked the stolen documents.
His identity was hidden in the interview where Gabriele used the codename "Maria" - but it was revealed by the journalist after Gabriele's arrest.
Nuzzi wrote a Twitter message on Saturday urging support for Gabriele saying: "Good luck, brave Paoletto, let's not leave him alone!"
The journalist has also said he is ready to appear before Italian authorities if there is a formal request from the Vatican that he be charged for receiving stolen goods -- something which has not happened.
The key question is whether Gabriele acted on his own or whether the leaks point to wider unease. That would spell a more serious crisis for the Vatican which is already struggling with paedophilia scandals and rising secularism.
Gabriele himself in his February interview spoke vaguely of "around 20" like-minded people spread across the Vatican institutions and there have been unconfirmed reports in the Italian press of battles between rival cardinals.
The butler expressed frustration with a culture of secrecy in the Vatican - from the mysterious disappearance of the daughter of a Vatican employee in 1983 to a quickly hushed-up double murder and suicide by a Swiss guard in 1998.
"There is a kind of omerta against the truth, not so much because of a power struggle but because of fear, because of caution," Gabriele said in the interview, using the term for the code of silence of the Sicilian mafia.
"It annoys people when you stick your nose in their dirty laundry," he said. The leaking of documents had been "a gesture of rage" against inaction, he added.
Gabriele comes across in the interview as a deeply religious man who says he was inspired by the Holy Spirit to reveal intrigues behind the Vatican walls so as to help the pope clear out corruption from the heart of the Catholic Church.
"There is a lot of hypocrisy, this is the kingdom of hypocrisy," he said.
Gabriele also said he was aware of the consequences of his actions but that the potential to change something in the Vatican was worth the risk.
"Being a witness to truth means being ready to pay the price," he said.