Santa Maria della Pietà al Colosseo is the Colosseum as a place of worship, since the Catholic Church still considers it to be consecrated.
The concordat between the Church and the Italian state allows for liturgical celebrations to be held in the amphitheatre on occasion, notably on Good Friday when the Pope leads the Stations of the Cross. When Mass is celebrated, a portable altar is used.
The Christian tradition is that many martyrs died in the Colosseum by being handed over to wild beasts during the games there in times of persecution, between its opening in AD 80 and the early 4th century. The most notable early martyr was St Ignatius, bishop of Antioch in Syria, who was arrested there and then taken to Rome to be killed in this way in AD 107. The tradition is almost certainly correct, but not one single contemporary record has survived of any martyrs being killed in the Colosseum.
The building was regarded throughout the Middle Ages as consecrated by the blood of martyrs.
In 1519 a tiny chapel was constructed next to the arena at its east end, with room for a priest and server to say Mass but not for a congregation. The doorway, with a broken triangular pediment, took up most of the façade. There was a small square window above it, just below the single-sloped roof.
Permanent depictions of the Stations of the Cross were also placed around the arena.
In 1670 Bernini published a proposal to build a full-sized domed Baroque church on the arena, but no work was done. Nothing was known then about the subterranean chambers under the arena, which would have prevented the proposal from being finished had it been started.
Both chapel and Stations of the Cross were removed in the 1870’s, when the monument was confiscated by the Italian state and archeological investigations started. Mussolini allowed religious ceremonies once again, as from 1926.
Hopkins & Beard: The Colosseum, Profile Books 2005. Picture of chapel on p168.