Christian paintings in the underground period were mainly concentrated in underground caves outside Rome. In the 9th century, when the church of Saint Sebastien was built, a group of such catacombs were dug up near the street in Abya district, with a total length of 100 kilometers and a width of about 0.9 meters. Several layers of small niches were chiseled on both walls to put the urns of the dead relatives of the Romans. From 1 century to the 5th century, Christians used it as a place for secret meetings, ceremonies, preaching teachings and burying martyrs. People call it the "catacombs" (Greek meaning "beside the concave ground"), and the murals in them can be traced back to 1 century at the earliest, with the most in the 3rd and 4th centuries. In order to hide people's eyes and ears, these paintings mostly use symbols and symbolic methods, and use some simple symbols to express religious content that only Christians can understand. For example, the cross represents the ascension of Christ, the palm tree represents martyrdom, and the anchor represents redemption. Inscriptions such as "Jesus Christ", "Son of God" and "Savior" are written next to the fish, representing Christ. The more complicated ones are The Good Shepherd in the Underground Tomb of San Brescia, The Virgin and the Prophet Isaiah, and The Prayer in the Underground Tomb of Winnie Maximi. It is often used to make relief with lime and then color it.
In these murals, we can see the connection between Christians and Greek and Roman myths and traditions, and they do not refuse to serve Christianity with the image of pagans. For example, in the picture of Christ's ascension to heaven, the image of Uranus, a god in Greek mythology, appeared at Christ's feet, and in the picture of Christ's baptism, the river god occasionally appeared. Hera Keres, a hero in Greek mythology, is used to symbolize the courage and strength of the Savior.
The color of murals in tombs is generally bright, which may be due to the dim underground light. However, out of belief and moral requirements, Christians often do not set colors according to the true nature, but try their best to make colors have their own spiritual connotation and give them certain symbolic significance. For example, in the mosaic of St. constanta's Church, St. Peter's skin is blue, which symbolizes the sky and the soul of heaven.
Underground caves used by Christianity were also found in many places outside Rome, such as Cumania, Sicily, Sardinia, Corsica, Tunisia and Algeria, which were all within the territory of the Roman Empire at that time. Those who are engaged in this kind of painting are not full-time painters, but believers from humble origins. Therefore, these paintings are all folk, and there are differences in local styles.