Why did Christians draw symbols on the catacombs?
Christians living in Rome during the 1st century were prohibited from having their own cemeteries. By the end of the century Chirstians in Rome began to erect a series of underground tunnels with a system of galleries along the walls to bury their dead. These 'catacombs' (word from Greek origin meaning 'near the hollow') were labeled as such for the the area along the Appian Way where the caves were located.
Early Christian symbols developed out of necessity. Christianity was considered strange by pagen Romans, moreover it was illegal to practice by the time of Nero. Therefore, because the early Christians lived in fear of retribution by Roman power, Christian symbols served as a sort of secret language. The symbols were a visible reminder of their faith as a spiritual reality. There were many symbols used to identify Christianity, four major ones are: the Good Shepherd, the orante, the monogram of Christ-interlocking Greek letters X and P, and the fish.