What shows a Christian iconography emerging, initially from Roman popular decorative art?
The Catacombs of Rome contain most of the surviving Christian art of the Early Christian period, mainly in the form of frescos and sculpted sarcophagi. They show a Christian iconography emerging, initially from Roman popular decorative art, but later borrowing from official imperial and pagan motifs.  Initially, Christians avoided iconic images of religious figures, and sarcophagi were decorated with ornaments, Christian symbols like the Chi Rho monogram and, later, narrative religious scenes. The Early Christians' habit, after the end of their persecution, of building churches (most famously St Peter's, Rome) over the burial places of martyrs who had originally been buried discreetly or in a mass grave perhaps led to the most distinctive feature of Christian funerary art, the church monument, or tomb inside a church. The beliefs of many cultures, including Judaism and Hinduism as well as classical paganism, consider the dead ritually impure and avoid mixing temples and cemeteries (though see above for Moche, and below for Islamic culture).
In the late Middle Ages, influenced by the Black Death and devotional writers, explicit memento mori imagery of death in the forms of skulls or skeletons, or even decomposing corpses overrun with worms in the transi tomb, became common in northern Europe, and may be found in some funerary art, as well as motifs like the Dance of Death and works like the Ars moriendi, or "Art of Dying".  It took until the Baroque period for such imagery to become popular in Italy, in works like the tomb of Pope Urban VIII by Bernini (1628–1647), where a bronze winged skeleton inscribes the Pope's name on a tablet below his enthroned effigy.  As cities became more crowded, bones were sometimes recovered after a period, and placed in ossuaries where they might be arranged for artistic effect, as at the Capuchin Crypt in Rome or the Czech Sedlec Ossuary, which has a chandelier made of skulls and bones.
surviving Christian art of the Early Christian period