A number of verses which were found written on the wall of a Mithraic temple in the Santa Prisca Church in Rome concern the doctrine of the Mithraic religion. But since these verses were only fragmentary, a complete understanding of the doctrine could not be discerned. Thus, we gain further knowledge through an iconographical study of the socalled accessory scenes of the bull-slaying action of the god Mithras. Accordingly, the results of my analysis of these scenes point to the following conclusion: 1) they represent a theogonic story which has nothing to do with Iranian religious literature; 2) the legend of Mithras (from birth to his and Sol's ascension into heaven by chariot) was incorporated in that theogony; and 3) the place of origin of the the accessory scenes - as well as the Mithraic theogony - can be placed in Late Hellenistic Phoenicia.