In the third arcade of the left nave, in place of the altar, one can admire the fresco of the Holy Trinity (1425-1426) by Tommaso Guidi, known as Masaccio.
In 1570 Giorgio Vasari covered the fresco with a stone altar and painting of the Madonna del Rosario now on show in the Bardi Chapel, upon the wishes of Camilla di Pietro Capponi, the widow of Pietro Arrighetti, in her last will and testament. The fresco was rediscovered in good condition during an 18th century restoration of the Church. It was removed from the wall and reassembled on the inner wall of the facade, between the central and the eastern portals. When another restoration was undertaken in 1952 the Death (or skeleton) painted by MASACCIO at the bottom of the Holy Trinity fresco was discovered underneath the plaster and was put back in its original place. On the ghoulish skeleton the words "WHAT YOU ARE, I ONCE WAS; WHAT I AM, YOU WILL BECOME" can be read. This is not a terrifying reminder of the transience of life that would ring somewhat macabre in this triumphal representation of the Holy Trinity. Instead they are the words that Christ, living on earth like men and women exiled for the sin of failing to submit to God, addresses to the Father and lives for eternity by submitting entirely to him, until his death. St. John tells us his words (John 17, 10-11), "I will remain in the world no longer, but they are still in the world, and I am coming to you. Holy Father, protect them by the power of your name—the name you gave me—so that they may be one as we are one." Humankind, therefore, secures the physical reality of his body, the temple of the Holy Spirit for eternal life and the certainty of the existence of God in the certainty of the Witness of the Risen One.
The magnificent architectural creation, one of the first and foremost examples of Renaissance perspective, still attracts students of art, as it did in the first half of the 1400s. In the scene of a chapel reminiscent of a classical triumphal arch overhead, one sees the Eternal Father, below, the Holy Spirit as a dove, and Jesus Crucified between the Virgin and St. John the Evangelist. On either side, below, are the figures of the work’s patrons, a husband and wife.
MASACCIO painted the fresco to the left of the axis of the wall, towards 12 o’clock, because the position from which he would have had an overall view, from afar – basically the portal that was recently reopened (see above) – has shifted to the right, northwards, on the opposite wall. Indeed, entering the church on this side, looking out over the entire space where the fresco is situated, one may also see the previous doorway, to the south, so that the Trinity would have been at the exact center of this wider field of vision between the pillars dividing the central from the side nave. This does not mean, however, that the triumphal representation of the Holy Trinity also assumes a closer central point of view, specifically by the tombstones lined up between these pillars. This dual field of vision strengthens the traditionally accepted view that it was BRUNELLESCHI who constructed this extraordinary perspective, according to experts who are aware of the studies the architect had made and the knowledge he possessed.