The main altar is reached by a wide presbytery and is flanked by a beautiful wooden choir.
Above is the great Crucifix, among the most beautiful of its period, it is the work of the French sculptor Guillaume Berthelot (1570-1648).
The altar painting, by Peter Paul Rubens, represents "Our Lady with the Child" surrounded by a choir of adoring angels.
The central part of the painting incorporates two separates image: the Our Lady with Child, the one by Rubens and the miraculous image of Santa Maria in Vallicella.
At the sides of the altar are two other famous paintings by Rubens.To the left, is the great piece dedicated to St.Gregory. To the right of the altar is that dedicated to the Saints Domitilla, Nereo and Achilleo.
Rubens painted these works with an innovative tecnique, not on canvas, but on slate, hoping to eliminate the reflection of light on the surface.
The tabernacle of gilded bronze is flanked by two angels in adoration and is crowned by a globe bearing a cross, below is the "Cor flammigerum" the flaming heart, the special emblem of St. Philip.