Between 1605 and 1608, Flaminio Ponzio constructed a building for the Canons of the Basilica. Its lower floor houses the new sacristy and the winter choir. The vault of the choir area was decorated by Passignano (1608-10) with frescoes depicting the Fathers of the Church and the prophets assembled around the Immaculate Conception. The marble high-relief of the Assumption of Mary is a masterpiece made by Pietro Bernini (1608-10), the father of Gian Lorenzo.
During the Pontificate of Leo XII (1823-29), the winter choir was transformed by Giuseppe Valadier into a baptistry. The Pontiff donated a basin to be used as a baptismal font for this area. It was made of red porphyry, which was enhanced by the surrounding marble balustrade.
Pope Adrian II welcomed Saints Cyril and Methodius to Saint Mary Major in 867 and gave his approval for the use of old Slav in the liturgy. This historical event is commemorated in the Baptistry by the plaques affixed by peoples of Slav languages.