"The Lord seemed to be sleeping. But I've always known that he was in that boat, that the boat was not mine or ours, but was his and he will not let it founder," Benedict said.
ANALYSIS: Resigning Pope Brings Doomsday Prophecy
He then thanked his cardinals and colleagues for "understanding and respecting" his decision to resign.
"In these past months I felt my strength of mind or body was diminishing...To love the church means also to have the courage to take difficult, painful decisions, always keeping the good of the church in mind, not oneself," Benedict XVI said.
Tomorrow he will be the first pope to leave the papacy by helicopter. At 8 p.m., the doors of the Vatican palace will close and the Swiss Guards who protect the head of the Catholic Church will go off duty.
Benedict will stay at the papal summer retreat at Castel Gandolfo, south of Rome, for about two months before moving to his new home, a former nunnery near the Vatican gardens.
"I won't have a life of travels, meetings, receptions, conferences, etc. I'm not returning to private life. I remain, so to speak, in the fencing of St Peter's, in the service of prayer, " Benedict said.