VATICAN CITY—Pope Benedict XVI will become the first pontiff in six centuries to resign, marking the end of a transitional papacy that focused more on theological and internal renewal and less on the broader challenges that face the Roman Catholic church at the start of its 21st century of existence.

The pope's surprise announcement paves the way for a successor who will confront anew the task of rebuilding the church's foundations in an increasingly secular and skeptical West while continuing to spread its roots in the rapidly growing emerging world.

The 85-year-old pope, who before his 2005 election was Cardinal Joseph Ratzinger, announced the decision to step down in a speech in Latin Monday to a small group of cardinals, saying he no longer had the vitality to perform his duties. Only two top Vatican cardinals were informed beforehand.

"The resignation itself is a real act of courage," said Terrence W. Tilley, head of the theology department at Fordham University in New York. "It is showing that he thinks of himself as a servant of the church, not some kind of superstar who should dominate it."

The resignation of Benedict, who heads a church of one billion world-wide, was emblematic of a pope who, though doctrinal in his teachings, often bucked traditions when it came to opening the Vatican up to the world beyond its medieval walls.