VATICAN CITY -- On Thursday, Pope Benedict XVI becomes the first pope to have resigned from the papacy in 600 years.
The 85-year-old theologian basked in an emotional send-off Wednesday from an estimated 150,000 people at his final general audience in St. Peter's Square. "I have taken this step with the full understanding of the seriousness and also novelty of the decision, but with a profound serenity in my soul," Benedict told the crowd.
Benedict has said he decided to retire after realizing he simply didn't have the "strength of mind or body" to carry on.
No date has yet been set for the start of the conclave of cardinals, who will vote in secret to elect Benedict's successor. The cardinals will have to decide whether it's time to look outside of Europe for a pope. The papacy was considered the realm of Italian prelates for centuries, until a Pole, John Paul II, was elected as pontiff in 1978, to be followed in 2005 by the German-born Benedict.
In one of his last papal tweets, Benedict wrote Sunday in English: "In these momentous days, I ask you to pray for me and for the church, trusting as always in divine providence."
Benedict will meet Thursday morning with cardinals for a final time, then fly by helicopter to the papal residence at Castel Gandolfo south of Rome.
There, at 8 p.m., the doors of the palazzo will close and the Swiss Guards in attendance will go off duty, their service protecting the head of the Catholic Church over - for now.
ABC News will provide live coverage by live coverage of the Pope’s departure from the Vatican at approximately 9:45 a.m. Central.