Pope Benedict XVI (search) will be formally installed Sunday, but his papacy began inside the Sistine Chapel (search) immediately after Cardinal Joseph Ratzinger was elected and responded with a simple: "I accept."

Hints of what that papacy will mean for the world's 1.1 billion Roman Catholics could come as early as Wednesday in his homily at Mass, which the Vatican said would be delivered in Latin, or during Sunday's installation at St. Peter's Basilica (search).

In the first homily of John Paul II after his election in 1978, the newly minted pope seized the moment to impress the faithful with his now-famous phrase: "Don't be afraid." John Paul directed it at all Catholics, but believers in his native Poland — then struggling to shake off communist rule — took his words especially to heart.

The Vatican describes the installation Sunday as "a solemn liturgical rite of inauguration." The ceremony used to be called a coronation back when popes wore crowns and wielded political as well as religious power.

Like just about everything at the Vatican, the installation of a pope is a carefully scripted affair drawing from centuries of tradition, yet bearing the subtle stamp of the new pontiff's style and sensibilities as he ceremonially takes up his cross.

Past papal installations have drawn world leaders, Catholic royalty and throngs of pilgrims to St. Peter's Square — all eager to size up a new pontiff and see him in action. The world will follow the event live on radio, television and the Internet.

It begins with the cardinals reciting the Lord's Prayer. Each of the scarlet-robed prelates then expresses his homage to the new pope, receives his embrace and exchanges the "Kiss of Peace."

In the past, cardinals bent down on one knee to express their homage, but in 1978, they stood as they expressed their devotion to Pope John Paul I, the "smiling pope" who died just 33 days after his election. The first John Paul ended the tradition of parading a new pope around the square on the papal throne and placing a three-layered tiara on his head.

The last pope to wear the tiara was Paul VI, who formally renounced the practice at the end of the 1962-65 Second Vatican Council, which modernized the church by allowing Mass to be said in local languages rather than only in Latin and made other sweeping reforms.

Historians say a single tiara was introduced in the early centuries of the church, and a second was added to show that the pope not only had spiritual authority but the power to crown the head of the Holy Roman Empire. For reasons not entirely clear, a third tiara was added in the 14th century, when French popes ruled from Avignon, France.

Among the most important parts of the Mass to bless Benedict is the placing of the pallium — a narrow stole of white wool embroidered with six black silk crosses — around his shoulders, pinning it in place with three jeweled gold pins. The pallium symbolizes his pastoral authority.

The Ceremony of Investiture will be performed by the senior cardinal deacon, Jorge Arturo Medina Estevez, the Chilean who on Tuesday evening proclaimed Benedict's name to the world from the basilica balcony.

In Latin, he will recite these words:

"Blessed be God, who has chosen you as shepherd of the universal church, entrusting you with this apostolic ministry. May you shine brilliantly during long years of earthly life, until, when called by our Lord, you are vested with immortality as you enter his celestial kingdom. Amen."

The liturgy ends with the pope giving his apostolic blessing, known in Latin as Urbi et Orbi ("To the city and the world").