The Vatican issued its most explicit decree so far against the ordination of female priests on Thursday, punishing them and the bishops who try to ordain them with automatic excommunication.
The decree was written by the Vatican's Congregation for the Doctrine of the Faith and published in the Vatican newspaper L'Osservatore Romano, giving it immediate effect.
A Vatican spokesman said the decree made the church's existing ban on female priests more explicit by clarifying that excommunication would follow all such ordinations.
Excommunication forbids those affected from receiving the sacraments or sharing in acts of public worship.
Rev. Tom Reese, a senior fellow at the Woodstock Theological Center at Georgetown University, said he thought the decree was meant to send a warning to the growing number of Catholics who favor admitting women to the priesthood.
"I think the reason they're doing this is that they've realized there is more and more support among Catholics for ordaining women, and they want to make clear that this is a no-no," Reese said.
The church said it cannot change the rules banning women from the priesthood because Christ chose only men as his apostles. Church law states that only a baptized male can be made a priest.
Proponents of women's ordination said Christ was only acting according to the social norms of his time.
They cite the letters of Saint Paul, some of the earliest texts of Christianity, to show that women played important roles in the early church.