If a priest is attending a Mass and not a celebrant at the Mass, he does not have to receive Communion. But if he is the celebrant, he must receive Communion. When the priest consecrates the bread and wine, the separation of the body and blood symbolizes Christ’s death. When the priest receives Communion, it symbolizes Christ’s Resurrection because his body and blood are reunited again in the person of the priest (who acts in persona Christi). These actions are important elements of the sacrificial aspect of the Mass. If a priest cannot receive Communion for whatever reason, then he should not celebrate Mass.
