Pope Leo XIV will formally begin his ministry as bishop of Rome with a liturgy steeped in tradition and rich in symbolism May 18.

The "Mass for the Inauguration of the Petrine Ministry," to be held in St. Peter's Square, will mark the visible and public start of his pontificate while grounding it in the Catholic Church's apostolic continuity.

Although the pope's canonical authority began the moment he accepted his election in the Sistine Chapel May 8, the installation Mass offers the universal church a moment of shared prayer and liturgical celebration to begin the pontificate.

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The morning Mass will begin with a ride through St. Peter's Square in the popemobile. But then the pope will enter the St. Peter's Basilica and proceed to the tomb of St. Peter, where, joined by the patriarchs of the Eastern Catholic churches, he will pause for prayer.

The symbols of the papal office -- the pallium, the fisherman's ring and the Book of the Gospels -- will have been placed there in advance, close to the remains of the apostle whose faith and martyrdom consecrated the church of Rome. Deacons then carry the objects into the square as part of the procession.

After the moment of prayer, the pope will join the main procession outside, and the liturgical celebration will begin at the altar set up on the steps of the basilica. The Mass will feature the singing of the "Laudes Regiae," an ancient litany invoking the assistance of Christ and the saints. The chant, together with the readings and prayers, frame the new pope's ministry within the communion of the church and the continuity of faith.

The Liturgy of the Word will include readings that recall St. Peter's call and mission, culminating in the account from St. John's Gospel in which the risen Christ tells St. Peter three times to tend to his sheep.

After the proclamation of the Gospel and before the pope's homily are the formal rites of installation, which take place in full view of the gathered faithful.

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The rites include the imposition of the pallium, conferred by a cardinal deacon, and the presentation of the fisherman's ring, given to the new pope by a cardinal bishop. A third cardinal, from the order of cardinal priests, will offer a solemn prayer invoking the Holy Spirit's strength for the pope's mission.

The pallium, a white woolen band embroidered with black crosses, represents the Good Shepherd who carries his sheep. It is worn by metropolitan archbishops and, in a particular way, by the bishop of Rome as a sign of his pastoral responsibility over the universal church. The fisherman's ring, bearing the image of St. Peter casting his nets, symbolizes the pope's role as successor to St. Peter and herald of the Gospel.

Following these rites, the pope will receive the gesture of obedience from 12 representatives of the people of God -- both clergy and laypeople -- from various parts of the world who reflect the diversity and unity of the church.

According to the liturgical program for the Mass released by the Vatican, the rites specific to the Petrine office have been carefully structured to precede the Eucharistic prayer, in keeping with reforms introduced under Pope Benedict XVI in 2013. This structure allows the distinctive symbols of the papal office to be celebrated in a dedicated moment, without interrupting the central structure of the Mass.

The Eucharistic prayer will follow, and the faithful will offer intercessions in multiple languages for the pope, the church, civil authorities, the suffering and the entire world. The Mass will conclude with the "Regina Coeli," prayed publicly in the square.

After the liturgy, Pope Leo XIV is expected to return inside the basilica to greet the heads of state and religious delegations assembled for the occasion. This greeting, conducted in the nave of the basilica in front of the Altar of the Confession, is not part of the liturgy but is an act of diplomatic and ecumenical respect. No bilateral meetings are expected to follow. The pope is not expected to pass among the faithful in the popemobile after the Mass.

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While the specific guests and delegations had not all been announced as of May 16, representatives of Christian churches, other religions and government leaders were expected to attend the Mass.

In the days following the installation Mass, Pope Leo will visit the major papal basilicas of Rome. He is scheduled to take possession of the Basilica of St. Paul Outside the Walls May 20; on May 25 he will take possession of the Basilica of St. Mary Major and the Basilica of St. John Lateran, the cathedral of the Diocese of Rome.