The spread of christianity brought a gradual change throughout hte Roman empire. Pagans and Christians lived together sometimes peaceably, sometimes not. Long periods of battle, persecution and dispute occurred until Constantine the Great made Catholicism the state religion in 324 a.d.
The first early Christian (Catholic) art was indistinguishable from Roman, except the content changed - subjects from the testaments - The sheperd, the fisherman etc. Catholic symbolism appeared on the walls of catacombs - the underground tombs which spread under Rome-. Commonly appeared symbols were the cross, the fish, the vine and the peacock. The early catholic portrait did not have the realism of the Roman portrait, but had a detached other-worldly look.
Architecture
The custom of burial in underground cemeteries (catacombs) was used in Rome long before Christianity took refuge in the catacomb in order to hold their congregation. The catacombs were of ten several levels deep wiht chambers off the passages in which the dead were intombed.
The Basilica church then developed from the line of the Etruscan temple and then the Roman Basilica which had been used as law court, money exchange and market.
Unlike the pagan temple which was sacred and only entered by priests, and viewed by plebians from the outside, the Catholic church was a place in which to assemble a large congregation of people. And so the interior of the church was highly decorated and the outside left plain. a rotunde was added as a pabtistry and the campanile or bell tower (eg: leaning tower of Pisa).
Painting
The walls of the catacombs were decorated wiht scenes from the testaments, executed in the manner of Roman fresco painting. The interiors of hte Bascillicas were decorated with carvings, frescoes and mosaics on wall and floor, depicting scenes from the bible.