Thomas Aquinas

“ It would seem that heretics and those who are cut off from the Church cannot confer Orders. For to confer Orders is a greater thing than to loose or bind anyone. But a heretic cannot loose or bind. Neither therefore can he ordain.
Objection 2: Further, a priest that is separated from the Church can consecrate, because the character whence he derives this power remains in him indelibly. But a bishop receives no character when he is raised to the episcopate. Therefore he does not necessarily retain the episcopal power after his separation from the Church. ”

“ The Pope, who has the fulness of episcopal power, can entrust one who is not a bishop with things pertaining to the episcopal dignity, provided they bear no immediate relation to the true body of Christ. Hence by virtue of his commission a simple priest can confer the minor Orders and confirm; but not one who is not a priest. Nor can a priest confer the higher Orders which bear an immediate relation to Christ's body, over the consecration of which the Pope's power is no greater than that of a simple priest. ”

“ When a man is raised to the episcopate he receives a power which he retains for ever. This, however, cannot be called a character, because a man is not thereby placed in direct relation to God, but to Christ's mystical body. Nevertheless it remains indelibly even as the character, because it is given by consecration.
Reply to Objection 3: Those who are ordained by heretics, although they receive an Order, do not receive the exercise thereof, so as to minister lawfully in their Orders, for the very reason indicated in the Objection. ”