From: 'St. Peter's Basilica - A Virtual Tour' by Our Sunday Visitor
Over the altar on the right is the "Incredulity of St. Thomas" by V. Camuccini, which replaces an earlier work on the same subject by Passignano. This work has a correct but rather cold neoclassical conception. At the time, Passignano was director of the Mosaic Studio, and was one of the most coherent Roman executors of David's work, but with more convincing results in historical subjects.

'The Altars and Altarpieces of New St. Peter's' by Louise Rice, ©1997, Cambridge University Press
The chapel of St. Thomas occupies the right niche in the south transept. The decoration of the walls and vault dates from the time of Clement VIII, who planned to dedicate the chapel in honor of St. John the Evangelist. Under Paul V, the body of St. Boniface IV was brought here from its tomb-altar in the old church. Thereafter, for a number of years, the chapel was officially referred to as the chapel of St. Boniface. The earliest documentary reference to the altar of St. Thomas is dated January 30, 1621.

The altar of St. Thomas was among the first altars in St. Peter's to be provided with an altarpiece during the pontificate of Urban VIII. The commission went to Domenico Passignano.... His altarpiece remained in place until around 1822, when it was replaced by a mosaic of the same subject, based on a design by Vincenzo Camuccini (1771-1844)

The story of the Doubting Thomas (John 20:24-29) was always very popular. Molanus advocated that all images of the apostle should illustrate this episode, and cited as his authority Pope Gregory the Great, who wrote: "The lack of faith of Thomas encourages us to faith more than does the faith of the believing disciples."

St Thomas altar in the Left Transept

St Thomas the Apostle altar area