The pope's butler has said he is innocent of stealing private correspondence but guilty of betraying the trust of the pontiff, whom he said he loved like a son would his father.
Paolo Gabriele entered the witness box in a Vatican courtroom to defend himself against a charge of aggravated theft.
Prosecutors say Gabriele stole papal letters and documents alleging power struggles and corruption and passed them off to a journalist in one of the most damaging scandals of the pope's reign.
In other evidence, the pope's private secretary, Monsignor Georg Gaenswein, said that he began having suspicions about Gabriele after he realised three documents that appeared in the journalist's book could only have come from the office he shared with Gabriele.
Gabriele faces four years in prison if he is found guilty, although most Vatican watchers expect he will receive a papal pardon if he is convicted.
Prosecutors have said Gabriele, 46, has confessed to leaking copies of the documents to Italian journalist Gianluigi Nuzzi, because he wanted to expose the "evil and corruption" in the church. They quoted him as saying in a June 5 interrogation that even though he knew taking the documents was wrong, he felt inspired by the Holy Spirit "to bring the church back on the right track."
Judge Giuseppe Dalla Torre asked Gabriele if he stood by his confession. Gabriele responded: "Yes."
Asked, though, by his lawyer Cristiana Arru how he responded to the charge of aggravated theft, Gabriele said: "I declare myself innocent concerning the charge of aggravated theft. I feel guilty of having betrayed the trust of the Holy Father, whom I love as a son would."
He insisted he had no accomplices, though he acknowledged that many people inside the Vatican trusted him and would come to him with their problems and concerns. He said he felt inspired by his faith to always give them a listen.
The trial opened over the weekend inside the intimate ground-floor tribunal in the Vatican's court tucked behind St Peter's Basilica. Dalla Torre has said he expects it to be over within three more hearings.