The crypt of the frescoes of the Basilica hosts a legitimizing episode for the church of Aquileia.
It depicts St. Peter while, sitting on the left, receives St. Mark, caught in a reverent attitude, and sends him to Aquileia.
The scene is set during the early days of Aquileian Christianity. From the historical point of view, there are no decisive clues about Mark's preaching in Aquileia.
Yet, during the Middle Ages, the link between Aquileia and the evangelist was never questioned, also for political reasons of conflict with Venice of which St. Mark is the patron saint.