For this reason, the basilica was once called St. Mary of the Manger and was also referred to as "the Bethlehem of the West."
This relic of the manger was historically kept in the crypt Chapel of the Nativity, which was first created in the seventh century but has been repeatedly reconstructed over the centuries.
Pope Sixtus V restored the crypt Chapel of the Nativity to the form seen today between 1585 and 1590 as he built the basilica's Sistine chapel.
In St. Mary Major's Sistine chapel, where the nativity scene is located, one can also see the first tabernacle to be made according to the precepts of the Council of Trent and an altar dedicated to St. Jerome.
Tradition holds that St. Jerome's relics were brought from Bethlehem to Rome shortly after the relic of the manger and are now located either in the main altar or this side chapel in the Basilica of St. Mary Major where the first nativity is currently displayed.
During the Advent and Christmas season this year, a relic of the manger has been brought out for veneration in the basilica's Borghese chapel, to the left of the main altar. This relic is usually found in the niche below the main altar. It was moved by Pope Pius IX from the Chapel of the Nativity to its current location under the altar in the 19th century.
Pope Francis visited the Basilica of St. Mary Major during Advent this year on the Solemnity of the Immaculate Conception, and offered Mass in the crypt Chapel of the Nativity.
Last year Pope Francis made a pilgrimage to the Italian town of Greccio during Advent to sign an apostolic letter on the meaning and importance of nativity scenes in the location where St. Francis of Assisi created the first nativity scene in 1223.
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The letter recalled the story behind St. Francis' first living nativity scene, or crèche. The saint asked a friend 15 days before Christmas to help him prepare "to bring to life" the memory of Christ's birth in Bethlehem.
"When St. Francis arrived, he found a manger full of hay, an ox and a donkey. All those present experienced a new and indescribable joy in the presence of the Christmas scene. The priest then solemnly celebrated the Eucharist over the manger, showing the bond between the Incarnation of the Son of God and the Eucharist."
"At Greccio there were no statues; the nativity scene was enacted and experienced by all who were present."
Thomas of Celano, the first biographer of St. Francis, wrote that someone present at the Mass had a vision of the baby Jesus himself lying in the manger.
Pope Francis wrote: "In a particular way, from the time of its Franciscan origins, the nativity scene has invited us to 'feel' and 'touch' the poverty that God's Son took upon himself in the Incarnation. Implicitly, it summons us to follow him along the path of humility, poverty and self-denial that leads from the manger of Bethlehem to the cross."
Photos of St. Mary Major Basilica by Courtney Mares and photo of Pope Francis by Vatican Media.