Listen to what the apostle urges us to do. I appeal to you, he says, to present your bodies as a living sacrifice. By this exhortation of his, Paul has raised every one of us to priestly status.
How marvelous is the priesthood of Christians for they are both the victims offered on their own behalf, and the priests who make the offerings! They have no need to go beyond themselves to seek what they are to immolate to God: with themselves and in themselves they bring the sacrifice they are to offer God for themselves. The victims remain and the priests remain, always one and the same. Immolated, the victims still live: the priests who immolate cannot kill. Truly it is an amazing sacrifice in which a body is offered without being slain and blood is offered without being shed.
Peter Chrysologus (AD 400-450), Sermo 108: PL 52, 500