SOUTH END -- St. Patrick provided a model of forgiveness when he returned to Ireland after escaping slavery there, auxiliary Bishop of Boston Robert F. Hennessey said March 17, at the St. Patrick's Day Mass at the Cathedral of the Holy Cross.

Speaking at the start of the Mass, the bishop said the faithful can emulate the forgiveness St. Patrick extended to a people who once enslaved him in their day-to-day lives. Irish pirates captured and sold the saint into slavery at age 16, but after his escape he returned to Ireland to spread the message of the Gospel.

"It was that act of forgiveness over sixteen hundred years ago that brings us here today, still bearing fruit," Bishop Hennessey said.

Bishop Hennessey was the principal celebrant of the Mass and blessed shamrocks for those present to take home as part of a Boston tradition. Msgr. Liam Bergin, a professor of theology at Boston College and a native of County Laois, Ireland, served as the homilist.

Msgr. Bergin spoke about St. Patrick, not as a cultural icon, but as a missionary immigrant walking a path of forgiveness in the name of Jesus Christ.

"Patrick did not come to Ireland to teach us how to perform Riverdance. He did not return to tell us that our identity was in our language, or in our music, or in our ways, or in our traditions, no matter how important or how beautiful they may be. He came to remind us that we are daughters and sons of God who are loved, brothers and sisters of Jesus Christ who are gifted with the Holy Spirit," he said.

Msgr. Bergin spoke of St. Patrick as an immigrant, a victim of what the modern world knows as human trafficking, but also as a teacher whose lessons on the Gospel transcend any singular place or culture.

He touched on one of the most common stories of St. Patrick in his teaching ministry -- the story of St. Patrick using the shamrock to explain the Trinity.

"The shamrock is a simple plant," Msgr. Bergin said.

"Rooted in the earth, it is used to point to things heavenly, representing the very nature of God -- the triune God -- Father, Son, and Spirit," he said.

Msgr. Bergin said St. Patrick, in crafting the legendary sermon, honed a teaching skill modeled by Jesus Christ himself.

"How often did Lord Jesus turn to the plants of the Earth to communicate the Good News? The mustard seed, the fig, the lilies in the field, the olive trees -- the soil and the seed is Jesus' image for the spread of the kingdom," he said.

The Mass itself highlighted cultural elements of a nation separated from a city by an ocean, but connected by a common point of heritage.

The prelude of the Mass included traditional Irish airs played on the harp by Kathleen Guilday, readings in Gaelic during the Liturgy of the Word, and bagpipes played before and after the Mass.

After the homily, Bishop Hennessey blessed the shamrocks on the steps of the altar in front of a statue of St. Patrick.

As he closed the Mass, Bishop Hennessey noted a common South Boston heritage between himself, cathedral rector Father Kevin J. O'Leary, and Boston Police Commissioner William Evans who attended the Mass alongside top Boston police officials and Mayor Martin J. Walsh.

"I'm real proud of our Irish heritage, and a lot of great people came from South Boston, so I'm glad that the bishop and Father O'Leary mentioned it. We've got long Irish roots, and great family. The Church meant a lot to us, so I'm very proud of the Church and my upbringing in South Boston," Commissioner Evans said.

Boston bagpiper Patrick McDonnell said he faced a busy schedule on St. Patrick's Day, after he played at the Mass, which he called a once in a lifetime opportunity.

"It's so beautiful inside there. To be part of a Mass like this is just a fantastic, beautiful Mass," he said.

Sandra Reyes, a native of Dublin, said singing of "Lady of Knock" during the Hymn of Thanksgiving reminded her of home.

"The school that I went to in Dublin was Our Lady of Good Council, so we learned to pray her prayer in Irish. We also learned that song," she said.