Fr. Gerald E. Murray, The Catholic Thing - Liturgical theology and law do not countenance that a bishop, let alone the diocesan bishop in his own diocese, concelebrate Mass with a priest as the principal celebrant (apart from a grave necessity, such as infirmity). This flows from the nature of the episcopal office: the bishop is the high priest in his diocese. He offers the sacrifice of the Mass for his people, while his priests, co-workers who serve the local Church under his authority, concelebrate with him.