The seven church circuit belongs to the machinery of the Roman Jubilee, and that machinery has a specific shape. Four of the seven, Saint Peter's, Saint Paul Outside the Walls, Saint John Lateran and Santa Maria Maggiore, are the papal basilicas, and each has a holy door that is walled up between Jubilees and opened by the pope at the start of one. Passing through the four doors is the core act of a Jubilee pilgrimage; the seven church walk is the older and more strenuous devotional form that grew around it.
Filippo Neri gave the circuit its settled shape in the 1550s as a deliberate counter attraction to Carnival, walking it with companions over a long day with readings, picnics in the vineyards and music. The music mattered: the gatherings at his Oratory produced a form of sung devotional drama, and the word oratorio comes from the room rather than from the genre.
The three non papal churches on the list are San Lorenzo fuori le mura, over the grave of the deacon Lawrence, Santa Croce in Gerusalemme, which holds the relics of the Passion brought from Jerusalem by Helena and stands on the site of her palace, and San Sebastiano fuori le mura on the Appian Way, above the catacombs.
For the Jubilee of the year 2000 the seventh church was changed. San Sebastiano was replaced in the official circuit by the Sanctuary of the Madonna del Divino Amore on the southern edge of the city, a modern shrine with a large local following. The substitution was widely noted and not universally welcomed, and the older list has continued in use alongside it, so there are now two versions of a circuit whose whole point was that it was fixed.