From the beginning, the Christian basilicas were adorned with mosaics or frescoes, at first in the apse, and very soon on the walls as well. Those in Gaul were lost together with the churches that housed them, and so we can only judge of them by contemporary descriptions and by surviving examples, especially in Italy, which was in close with Gaul at this time.
The iconography of the 4th century is rather close in style to the realism of later Roman painting, although by the end of the century, even in Rome, it is already changing towards the Byzantine style; in content it combines themes from the symbolic paintings of the catacombs (Christ as the Lamb, the Good Shepherd, etc.) with scenes from the Old and (more and more with time) from the New Testament. The basilica of St. Ambrose in Milan, dedicated in 386, contained frescoes (as we know from the inscriptions of the saint himself) from the Old Testament, and the following ones from the New Testament: the Annunciation, the conversion of Zacchaeus, the woman with the issue of blood, the Transfiguration, and St. John leaning on the breast of the Savior. Judging from the contemporary mosaics at St. Pudentiana in Rome, the style of these icons was already very close to the later Byzantine style. In the Basilica of St. Paulinus in Nola (404), the two sides of the nave contained scenes from the Old and New Testaments, and in the space between the windows above were apostles and saints, with Christ the King in the apse. There was yet no fixed rule for the depiction of various feasts or scriptural events, and there was no formal canonization of the saints who might be portrayed in icons; apostles, martyrs, and even recent bishops and ascetics were depicted according to their local veneration. There is even a case where, in the baptistery of the monastery of Sulpicius Severus at Primuliacum in southern Gaul, the recently-reposed St. Martin is depicted on one wall, and the still-living Bishop Paulinus of Nola on the opposite wall – something which aroused the good-natured protest of St. Paulinus, who wrote Severus: “By depicting me alone on the opposite wall, you have contrasted my lowly figure, shrouded in mental darkness, with Martin’s holy person.” (St. Paulinus, Letter 32)
The distinctive Byzantine style is already evident in the 5th century, and the 6th century is the age of an already developed and perfected art. The great basilicas of Ravenna are monumental triumphs of Byzantine iconography – an art which in style and subject-matter has not changed essentially through the ages, and is still very much alive today. The Byzantine style was universal in the Roman Empire, as may be seen in the icons even of the remote border area of Mt. Sinai, where the mosaic of the Transfiguration in the apse is identical with later icons of the feast down to our day. This is the Christian art that was known to the great Western hierarchs of the 6th century, St. Gregory, Pope of Rome and St. Gregory of Tours.
In Gaul, mosaic icons are known (History of the Franks 2.16; 10.45), but more commonly we hear of frescoes. The original basilica of St. Martin had frescoes which were restored by St. Gregory, as he himself relates (HF 10.31): “I found the walls of St. Martin’s basilica damaged by fire. I ordered my workmen to use all their skill to paint and decorate them, until they were as bright as they had previously been.” These frescoes must have been impressive, for when treating of the stay of a certain Eberulf in the basilica (under the law of sanctuary which then prevailed), St. Gregory writes: “When the priest had gone off, Eberulf’s young women and his men-servants used to come in and stand gaping at the frescoes on the walls” (HF 7.22). St. Gregory has preserved for us a brief account of how the frescoes were painted (5th century): “The wife of Namatius built the church of St. Stephen in the suburb outside the walls of Clermont-Ferrand. She wanted it to be decorated with colored frescoes. She used to hold in her lap a book from which she would read stories of events which happened long ago, and the tell the workmen what she wanted painted on the walls” (HF 2.17). This “book” might have been the Scriptures, the Life of a saint, or even, as Prof. Dalton suggests, “some sort of painter’s manual like those used in the East” (vol. 1, p. 327).
When restoring the main basilica of Tours (distinct from the basilica where Martin’s relics reposed), as Abbot Odo informs us precisely in his life of St. Gregory (ch. 12), the latter “decorated the walls with histories having for subject the exploits of Martin.” It so happens that we have a list of these iconographic scenes in a poem of Fortunatus describing the basilica (Carmine 10.6). They are: (1) St. Martin curing a leper by a kiss; (2) dividing his cloak and giving half to a beggar; (3) giving away his tunic; (4) raising three men from the dead; (5) preventing the pine tree from falling on him by the sign of the Cross; (6) idols being crushed by a great column launched from heaven; (7) St. Martin exposing a pretended martyr. We can only regret the disappearance of such a notable monument of Orthodox Christian art, just one of many in 6th-century Gaul, the likes of which were not seen in later centuries in the West (where the Roman-Byzantine style was gradually lost); but we may gain a general idea of its appearance in the contemporary basilicas of Ravenna with their mosaic icons. One of these basilicas, indeed, was dedicated originally to St. Martin of Tours, the dedication later being changed to Sant’ Apollinare Nuovo.
Separate panel icons also existed at this time. In the history of Bede it is stated that St. Augustine of Canterbury and those with him, after landing in Britain in the year 597, came to King Ethelbert of Kent “bearing a silver cross for their banner, and the image of our Lord and Savior painted on a board” (Ecclesiastical History of England, Book I, ch. 25). In the Life of the Fathers (12.2) we read of “the icons (Latin iconicas) of the apostles and other saints” in the oratory where St. Bracchio prayed. It should be noted that the oratories and small village churches of Gaul would not, of course, be in basilica style or usually made of stone; they were generally of wood, and the icons in them were painted on boards and hung on the walls. The most detailed reference to these 6th-century panel icons is in St. Gregory’s Glory of the Martyrs (ch. 22), where we read, in the account “of the Jew who stole an icon (Latin iconica, or in one manuscript, icona) and pierced it,” the following which is also an impressive testimony of the truly orthodox attitude of the Church of Gaul at that time, as contrasted with the iconoclast sentiment which seized part of Gaul (as it did also of the Christian East) in the century of Charlemagne. Here are St. Gregory’s words: “The faith which remained pure among us up to this day causes us love Christ with such a love that the faithful who keep His law engraved in their hearts wish to have also His painted image, in memory of His virtue, on visible boards which they hang in their churches and in their homes… a Jew, who often saw in a church an image of this sort painted on a board (Latin imaginem in tabula pictam) attached to the wall, said to himself, ‘Behold the seducer who has humiliated us’… Having come then in the night, he pierced the image, took it down from the wall, and carried it under his clothes to his house in order to throw it into the fire.” He was discovered when it was found that the image shed copious blood in the place where it had been pierced (a miracle which occurred also later in Byzantium with the Icon of the Iviron Mother of God, and in Soviet times in Kaplunovka in Russia with a crucifix).
A number of such panel icons on wood have come down to us from 6th-century Mount Sinai; they are identical in appearance to the icons which pious Orthodox Christians cause to have painted for their churches and homes even today. (Vita Patrum: The Life of the Fathers by St. Gregory of Tours. Introduction to Orthodox Gaul, pp. 80-82)