THE 17th Ecumenical Council of the Church was convened by Pope Martin V, first at Basel (1431), and then, after his death, Pope Eugenius IV transferred it to Italy, first to Ferrara (1438-39) joined by the Byzantine emperor John VIII Palaiologos, and then to Florence when plague hit the duchy of Ferrara. After four years there, it adjourned to Rome, closing in 1445.

The purpose was to bring about reconciliation between the Churches of the Orthodox East and the Latin West after the Great Schism of 1054, when political and ecclesiastical differences had led to the fracturing of the Christian Church. The Union of Florence (1439) healed the division, although, after the fall of Constantinople (1453), it withered on the vine; in 1484, unity was lost.

The Council had much to say about the primacy of the Bishop of Rome and the doctrines of purgatory and in particular the much disputed place of the Holy Spirit in the Creed; the so-called “procession of the Holy Spirit”.

Does the Holy Spirit come only from the Father, as is the Orthodox tradition, or from “the Father and the Son”? The additional clause in Latin, “filioque” (“and the Son”), gives its name to the dispute.

From the point of view of the Latins, the filioque clause can be grounded in holy scripture by the texts of both John 16.13-15 and Romans 8.9 and in Philippians 1.19. It can be argued that it lies behind the sending of the Holy Spirit by Jesus in the Johannine texts of John 14.16, John 15.26 and 16.7.

One of the Italians in Florence who could have seen Eastern hierarchs and clergy in the streets and learned of their debates was the short-lived painter Francesco di Stefano (1422-57), who is called Pesellino, using a diminutive nickname of his grandfather’s name, the painter Pesello, who brought him up there.

This exhibition, 600 years after his birth, is his first monograph show. It is ablaze with colour and rich in detail (had he also worked with goldsmiths?), and the panels and devotional pictures, including a delightful diptych of the Annunciation from the Courtauld Gallery, are enlivened with Tuscan light.

In 1455, Pesellino was commissioned for an altarpiece in St James’s, Pistoia, some 40km west of Florence. The detailed contracts drawn up by the confraternity of priests for the Trinity altarpiece survive, making it possible to trace the vicissitudes of one of the few known works of this much admired artist, finished at his death by Fra Filippo Lippi (c.1406-69).

© Photo The National Gallery, LondonFrancesco Pesellino, Fra Filippo Lippo and Workshop, The Trinity (1455-60), from The Pistoia Santa Trinità Altarpiece  

The Pistoia altarpiece rightly dominates the room (184.5 × 181.0cm). It centres on an enthroned Father in whose outstretched hands are held a T-shaped cross on which Jesus slumps forward, his eyes sealed in death. Above his head, as if about to nest on his halo, is a dove representing the Holy Spirit. Behind the cross, a river runs through the Tuscan hills. Ultimately, the design for the Godhead derives from earlier Gothic sculpture.

The contract specified four saints to stand, two by two, at either side of this. Three were readily suggested; the Apostle St James (patron saint of Pistoia and of the church); the martyr St Zeno, from Mauretania, who was patron saint of the clergy of Pistoia, and St Jerome, who had had a vision of the Trinity.

Choice of the fourth saint provoked debate in the confraternity, but ultimately was to favour St Mamas, from Caesarea in Cappadocia (modern-day Kayseri in central Anatolia), who was born in prison shortly before his illustrious parents were martyred for their faith.

Aged just 15, he was mistaken for a shepherd when he, too, was arrested as a Christian; he had taken to living as more or less a hermit. Not for him the latest courtly dress of the Medici court in which he is garbed. Maybe he dressed up for his trial before the emperor Valerian.

Art historians puzzle over the inclusion of Mamas, as his iconography is reasonably uncommon in Western art, even though his relics were taken to the safety of France during the eighth century Muslim invasions of Anatolia. His shrine is in the cathedral at Langres, for which the French Renaissance artist Jean Cousin (c.1500-60) later designed some fine tapestries depicting his life.

There is a neat balance established in the four surviving smaller narrative scenes in the predella below: both Jerome and Mamas befriend a lion, the scholarly Bible translator after removing a thorn from its foot (a Christian appropriation of the story of Androcles) and the imprisoned noble youth with those to whom he was to be thrown.

But the prominence given to Mamas in the altarpiece belies such a simplistic link, as it overlooks the widespread popularity of the cult of the teenage saint in Eastern Orthodoxy, especially across Lebanon. Painted some ten or 12 years after the Council of Florence, and in the spirit of a reunited Church on earth, the Trinity altarpiece can be read as contemporary propaganda.

Such redolence would be more readily apparent if the missing central panel from the predella (all by Filippo Lippi) could have been loaned to the National Gallery from St Petersburg. Sadly, that is currently not possible.

Between the four beautifully depicted scenes of St Mamas in prison, the beheading of St James, St Zeno exorcising the daughter of the Emperor Gallienus (Valerian’s son and co-emperor), and St Jerome, there was a fifth, wider, panel that showed the vision of St Augustine.

© RMN-Grand Palais (musée du Louvre) / Daniel ArnaudetFrancesco Pesellino, Adoration of the Shepherds, pen and brown ink, coloured wash, gold highlights, on loan from the Musée du Louvre, Paris (inv. 1434 recto)  

St Augustine sees the Christ child spooning water from the sea into a hole in the ground, which in Lippi’s painting is a small river heading off into the Tuscan hills.

The Christ-child assured Augustine that it was even more futile to attempt to understand the Trinity than to try to spoon the water out of the river into a hole in the ground. The pious story is not in Augustine’s own writings.

To show that some texts do get added in translation, it is first found interpolated in the English version of the widely popular Golden Legend by the Dominican Jacobus de Voraigne (mid-13th century), printed by William Caxton in 1483. There, it was said to be drawn from the subject of an Augustinian altarpiece in Antwerp; a generation before, our altarpiece was already complete.

So, the reunification the Latin and Greek Fathers achieved by the Council of Florence may well have prompted the commission from the confraternity in September 1455 and is given meaning by the inclusion of the novel scene of St Augustine.

When Pesellino died (29 July 1457), the commission had not been completed, and the painting, after a dispute with how much the family should be paid for his work, was entrusted to Filippo Lippi to complete for Bishop Donato de Medici to dedicate in June 1460. He greatly admired it.

St Mamas, in his fetching tunic hitched up to his waist, is the only one of the saints to look at us, with that adolescent gaze of challenge and self-assurance, while St Jerome opposite surely shows us how we should approach the Godhead, by reflection (his right hand is on his heart) and allowing ourselves to be absorbed by the vision of Christ on the Cross as the Trinity makes known God’s love for us.

Other works also betray memories of the Byzantine emperor’s visit to Florence in 1438/9, most fittingly in the headgear of the Emperor Constantine in The Miracle of St Sylvester (Worcester Arts Museum, Worcester, Mass.) and among those sailing with Melchior to pay homage to the Christ-child (Clark Art Institute, Williamstown, Mass.). A lost world of delights.

“Pesellino: A Renaissance Master Revealed” is in Room 46 at the National Gallery, Trafalgar Square, London WC1, until 10 March. Phone 020 7747 2885.

www.nationalgallery.org.uk