In ancient Rome the burial ground along the sides of the main streets was very expensive, so only the rich
could afford individual or family tombs, while the poorer classes joined in burial associations use to buy the ground out of the city, purchased in common, in a vertical direction, digging tunnels in tuff and burying people layer after layer.
This was the method for Christians, pagans and Jews. Many catacombs, initially pagan, with the diffusion of Christianity were transformed into places of Christian burial.
The Romans were not religious fanatics and as pagans they considered other cults, especially in late antiquity, with a tolerant attitude. In most cases, Christians were not persecuted because of their faith, but for the way in which they professed: refusing to be loyal to the state and to venerate as divinity (given that were monotheists) the emperor.
Among the Christians of the catacombs the cult of martyrs as we understand it today was widespread only in exceptional cases and, normally, for the religious they were not using the catacombs uncomfortable and dark, even during the period of persecution, but in the home of some respectable member of the Community.
In the early Middle Age, the catacombs became places of worship and shrines. Among the nineteen known catacombs of Rome is better to initially visit those of San Callisto and San Sebastiano, situated along the via Appia and close one to another.
San Callisto: These catacombs dates back to 170 AD and have a vast labyrinth of tunnels, the overall length of which exceeds 20 km. At the origin they were four different burial places, which were united in a second time. Next to the Crypt of the popes there is the tomb in which Pope Pasquale I found the rest of the body of Santa Cecilia.
San Sebastiano: even the origins of this burial place dates back to the times of paganism; it is possible to visit the tomb of Saint Sebastiano who was martyred in 303 A.D., during the period of persecution of Diocletian. In the triclinium, used for the ritual lunch funeral, some engravings recall that at the time of the Emperor Valerian there were exposed the remains of the Apostles Peter and Paul who remained here for forty years. On this place has been also built the Basilica di San Sebastiano, that until the IX century took the name of the two Apostles. The basilica, which can be visited together to the catacombs, dates back to the IV century, but was completely renovated in 1609.