Early Christianity was suffused with Greek thought and, more generally, with certain elements of Greco-Roman culture. Images of one sort or another played a very important part in this culture, flatly contradicting the prohibition on graven images set out in the Old Testament. As Christianity spread, gaining more converts in the pagan world, it inevitably started to reflect some of the cultural assumptions of pagan culture. And the use of images was at the heart of this.
However, in the ensuing synthesis between pagan culture and early Christianity, certain images were still strictly forbidden in fine art and sculpture. Though the sarcophagi of wealthy Roman Christians were adorned with Christian imagery, they were entirely free of representations of Christ, which were considered idolatrous. To be sure, themes of death and resurrection could still be depicted in art—such as the story of Jonah and the whale—but direct representation of the crucifixion and resurrection of Christ was deemed unacceptable for the reasons already given.
At this stage of its existence, Christianity was still very much a mystery religion. And the crucifixion and resurrection of Christ were part of the mystery. This precluded them from being given direct representation in works of art. Nonetheless, the penchant of wealthy pagans for elaborate imagery meant that early Christian art could still display considerable richness and sophistication, just not in its depiction of Christ.