The lower register of the imagery presents the sacrifice of Isaac (detail), with Abraham and his son in the center (detail), the ram on the left (detail), and the fiery altar on the right (detail).

Above this scene we see on the left edge St. Blaise above Melchizedek (detail) and on the right edge St. Nicholas above Aaron (detail).

In the center of the upper register is the Traditio Legis (detail).

The imagery thus establishes the typological background as well as the divine authority behind the eucharistic celebration for which the altar is intended.  Abraham's willingness to sacrifice Isaac is a type of God's giving his own only son in the sacrifice that the Mass remembers.  Aaron and Melchizedech are types of the priesthood that will come under the new covenant, represented by the bishops St. Nicholas and St. Blaise, whose partake of the authority that Christ gave to the apostles, represented here by the Traditio Legis, the giving of the Law and the Keys to St. Paul and St. Peter respectively.

More of Abraham
More of Melchizedek
More of Aaron
More of St. Blaise
More of St. Nicholas
More of the Traditio Legis

Photographed at the site by Richard Stracke