Before I start explaining the artwork I should give a little context to the form itself which is a sarcophagus. This was the preferred type of funeral box among Romans and before them among their Etruscan ancestors. The word "sarcophagus" actually means "eater of flesh" and was a stone box chosen because of the stone's mineral ability to hasten decay, much like the spreading of lye and lime would do. So, a sarcophagus has similarities to an ossuary but it also has similarities to permanent monuments, which is ironic considering the less than permanent approach to the encased remains. That said, the image below is the sarcophagus of Junius Bassus.
Perhaps a little more contextual history is in order. Christianity was an outlawed religion in its first three centuries -- more so than to merely say that it was not a lawful and recognized religion, it was the subject of intense persecution. The previous three emperors had crucified Christians by the hundreds perhaps thousands, enslaved and imprisoned others, martyrdom and torture were common, and it was so politically vilified that a politician such as Junius Bassus probably kept his conversion secret until his death. Christianity only became legal and somewhat safer in A.D. 312 when Constantine signed The Edict of Milan declaring Christianity to be his religion and a lawful Roman religion.
That would have occurred around the time that Junius Bassus was born into a political family of high aspirations. Such people were often recognized in Rome with sculpture busts, many lining the Appian Way and the Roman Forum, and he would have expected a public memorial after death placed on view somewhere such as an atrium or in the senate. That is why his sarcophagus is so elaborately sculpted and that is why it is remarkable.
What Junius Bassus chose to do was to write large his beliefs for public viewing at a time when such things did not exist. A.D. 359 was only 30 years after the first Nicean Council and the beginnings of codifying Christian beliefs, and centuries before churches and cathedrals would become covered with such carvings and imagery. The artist may or may not have been a Christian but was certainly a Roman and acquainted only with Greek and Roman symbols and means of telling a story.
Yet the sculptor and Junius Bassus chose to tell us some of the foundational beliefs and Bible stories in a remarkably articulate way. Adam and Eve in the Garden and their Fall to Original Sin, Jesus entering Jerusalem for the Passion Week of His crucifixion, Daniel in the Lions' Den, Christ transfigured glorified and enthroned, Abraham and the sacrifice of his son Isaac, and other Bible stories and doctrines that Junius Bassus chose to declare to the public in stone, at a time when many of those doctrines had not yet been rendered in universities and seminaries. Yet he knew and understood them, and the sculptor conveyed them so clearly that we can easily parse through them nearly two millennia later.
One of the most striking images is that of Christ Enthroned. How was a Roman artist to depict that subject? It is not the same as showing Apollo, or substituting Sol Invictus, the image of Apollo in his chariot as a secret symbol appropriated for Christ. The artist had to convey the concepts of "son of God", the son in power, the transfiguration and glory as described in the gospels, and the sense that the two prophets flanking Jesus worship Him and respect His authority. And the only way a Roman sculptor could visualize God was as Zeus, father to the Roman deities.
You can see Zeus thus depicted on the breastplate of Caesar Augustus in the Primaporta statue (yes, that is the same Augustus mentioned in Luke's gospel account of Jesus's birth, the one who declared that all the world should be taxed):
One has to remember that all the detailing -- all of the detailing preceded such decoration in churches. The columns, dentils, etc., were features of Roman temples, triumphal arches, and monuments of governmental power at the time that these sculptures were carved.
One also has to remember that this was done for a largely literate public, one prone to discourse and debate, at a time when Greco-Roman culture flourished and dominated the known world.