Apostle Paul visited the religious and cultural experience of the Greco-Roman world through cities like Ephesus, Corinth, Thessalonica, and Rome; this lead him to the brazen act of spreading christianity beyond Palestine. In Rome, there were an innumerable amount of religions that were assimilated after a people's conquer; this was due to the fact that polytheism, in Rome, was accreditted. Eventually, the Pantheon was rebuilt by Emperor Hadrian in the early second century in honor of his building program that pushed architects to build new and better landmarks as well as sculptures from artisans; also being Emperor, he was tasked with making temples. Easily distinguished Christian art is traceable to late of the second century, and early of the third. Although, the Old Testament writes off graven images, this did not defer future artisans from continuing their Greco-Roman culture, which was heavily based off of imagery, buildings, sculptures, and etc. As the religion, Christianity, continued to expand and gain more converts, many of whom were raised to appreciate images from previous religions, naturally wanted to continue their way of doing so. For example, there was a change in burial practices in the Roman world, and I quote, "Outside the city walls of Rome, adjacent to major roads, catacombs were dug into the ground to bury the dead. Families would have chambers or cubicula dug to bury their members. Wealthy Romans would also have sarcophagi or marble tombs carved for their burial; The Christian converts wanted the same things. Christian catacombs were dug frequently adjacent to non-Christian ones, and sarcophagi with Christian imagery were apparently popular with the richer Christians." A conspicuous aspect of the Christian art, noted in the third century, is the leave of direct imagery; by that I mean during the early period there were no images based off of Christ's Resurrection or Crucifixion. However, the themes were still there in abundance; "For example the story of Jonah being swallowed by a great fish and then after spending three days and three nights in the belly of the beast is vomitted out on dry ground was seen by early Christians as an anticipation or prefiguration of the story of Christ's own death and resurrection. Images of Jonah along with those of Daniel in the Lion's Den, the Three Hebrews in the Firey Furnace, Moses Striking the Rock, among others are widely popular in the Christian art of the third century both in paintings and on sarcophagi." One can effortlessly visualize how empowering those images and works of art could be for those persecuted. Their faith and prayers must have been answered, because an event soon unfolded that changed everything in Rome dramatically... In 312, the Emperor Constantine defeated his principal rival Maxentius at the Battle of the Milvian Bridge. Accounts of the battle describe how Constantine had seen a sign in the heavens portending his victory. Eusebius, Constantine's principal biographer, describes the sign as the Chi Rho, the first two letters in the Greek spelling of the name Christos. After that victory Constantine became the principal patron of Christianity. In 313 he issued the Edict of Milan which granted religious toleration. Despite Christianity not becoming the official religion of Rome until the end of the fourth century, Constantine's imperial sanction of Christianity transformed its status and nature. Prior to his rule, there was nothing to distinguish churches from domestic architectures, other than the joining of two rooms to create a room of service. For centuries the Ruler was tasked with construction of temples throughout the Empire of Rome; so it was no surprise that Constantine would want to create a temple in honor of Christianity; which was more challenging than not. The typical layout for a temple would be quite inappropriate, and so they eventually went with a basilica; audience halls that were part of imperial palaces.
Now after displaying some knowledge of Early Christianity before and after Constantine we shall get to the meat. These two pictures are of the Apostle Paul, one before Constantine and the other is after; he looks considerable different. On the left (before), he is bald, looks a little older and is meant to be seen from one direction. Whereas the picture to the right (after) seems to be a bit more younger and in shape, he has more of the ideal look that we've seen in previous sculptures; as well as being visible from all sides. The details in the right picture is vastly superior to its left counterpart, however the picture on the left has more colors to it. So, if one were to compare and contrast the two eras of Christian art based off of these alone I'd say, prior to Constantine, the art work wasn't given as much attention because of Christianity being privy; which leads to the images being less endorsed such as after Constantine. In summary, before-Constantine, archeticture and art work for Christianity lacked the direct imagery and lavished churches as after-Constantine.