From early shrine to Chiesa Nuova
The roots of the site go back to the late 6th century AD, and devotion centered on the image of the Madonna della Vallicella long before the current church existed. After the older medieval building gave way to the Oratorian rebuilding that began in 1575, the present church was consecrated in 1599 and its facade completed in 1605. That is why the Roman nickname Chiesa Nuova still makes sense even though the devotion here is much older.
What San Filippo Neri changed here
In 1548 San Filippo Neri founded the Confraternity for Pilgrims and Convalescents, and the church became inseparable from his Rome. When Pope Gregory XIII entrusted Santa Maria in Vallicella to the Oratorians, the site turned from a local cult church into the spiritual center of a movement built on charity, preaching, and practical care for pilgrims. That backstory is one reason the place still feels lived-in rather than merely monumental.
Look up for Pietro da Cortona
Do not let the side chapels steal your attention too early. The main nave, dome, and apse carry the great cycle by Pietro da Cortona, and the architecture is arranged to make that upward sweep read as one continuous performance. From the center of the church, the interior feels far more theatrical than its fairly plain exterior prepares you for.
Do not rush the Rubens high altar
The high altar is the church's signature trick. In 1608 Pieter Paul Rubens created the slate-panel altarpiece as a protective cover for the older miraculous image, turning devotion, engineering, and Baroque showmanship into one object. If you catch the mechanism in use on a Saturday or major feast, it feels wonderfully Roman; if you do not, the altar is still the key stop that explains the whole church.