in no case can they lawfully consecrate the Eucharist,
or confer the other sacraments.

Reply Obj. 3: The priest, in reciting the prayers of the mass, speaks
instead of the Church, in whose unity he remains; but in consecrating
the sacrament he speaks as in the person of Christ, Whose place he
holds by the power of his orders. Consequently, if a priest severed
from the unity of the Church celebrates mass, not having lost the
power of order, he consecrates Christ's true body and blood; but
because he is severed from the unity of the Church, his prayers have
no efficacy.
_______________________

EIGHTH ARTICLE [III, Q. 82, Art. 8]

Whether a Degraded Priest Can Consecrate This Sacrament?

Objection 1: It seems that a degraded priest cannot consecrate this
sacrament. For no one can perform this sacrament except he have the
power of consecrating. But the priest "who has been degraded has no
power of consecrating, although he has the power of baptizing" (App.
Gratiani). Therefore it seems that a degraded priest cannot
consecrate the Eucharist.

Obj. 2: Further, he who gives can take away. But the bishop in
ordaining gives to the priest the power of consecrating. Therefore he
can take it away by degrading him.

Obj. 3: Further, the priest, by degradation, loses either the power
of consecrating, or the use of such power. But he does not lose
merely the use, for thus the degraded one would lose no more than one
excommunicated, who also lacks the use. Therefore it seems that he
loses the power to consecrate, and in consequence that he cannot
perform this sacrament.

_On the contrary,_ Augustine (Contra Parmen. ii) proves that
"apostates" from the faith "are not deprived of their Baptism," from
the fact that "it is not restored to them when they return repentant;
and therefore it is deemed that it cannot be lost." But in like
fashion, if the degraded man be restored, he has not to be ordained
over again. Consequently, he has not lost the power of consecrating,
and so the degraded priest can perform this sacrament.

_I answer that,_ The power of consecrating the Eucharist belongs to
the character of the priestly order. But every character is
indelible, because it is given with a kind of consecration, as was
said above (Q. 63, A. 5), just as the consecrations of all other
things are perpetual, and cannot be lost or repeated. Hence it is
clear that the power of consecrating is not lost by degradation. For,
again, August