Objection 1. It would seem that those who are schismatics, heretics, excommunicate, suspended or degraded have the use of the keys. For just as the power of the keys results from orders, so does the power of consecration. But the above cannot lose the use of the power of consecration, since if they do consecrate it is valid, though they sin in doing so. Therefore neither can they lose the use of the keys.
Objection 2. Further, any active spiritual power in one who has the use of his free-will can be exercised by him when he wills. Now the power of the keys remains in the aforesaid, for, since it is only conferred with orders, they would have to be reordained when they return to the Church. Therefore, since it is an active power, they can exercise it when they will.
Objection 3. Further, spiritual grace is hindered by guilt more than by punishment. Now excommunication, suspension and degradation are punishments. Therefore, since a man does not lose the use of the keys on account of guilt, it seems that he does not lose it on account of the aforesaid.
On the contrary, Augustine says (Tract. cxxi in Joan.) that the "charity of the Church forgives sins." Now it is the charity of the Church which unites its members. Since therefore the above are disunited from the Church, it seems that they have not the use of the keys in remitting sins.
Further, no man is absolved from sin by sinning. Now it is a sin for anyone to seek absolution of his sins from the above, for he disobeys the Church in so doing. Therefore he cannot be absolved by them: and so the same conclusion follows.
I answer that, In all the above the power of the keys remains as to its essence, but its use is hindered on account of the lack of matter. For since the use of the keys requires in the user authority over the person on whom they are used, as stated above (17, 2, ad 2), the proper matter on whom one can exercise the use of the keys is a man under one's authority. And since it is by appointment of the Church that one man has authority over another, so a man may be deprived of his authority over another by his ecclesiastical superiors. Consequently, since the Church deprives heretics, schismatics and the like, by withdrawing their subjects from them either altogether or in some respect, in so far as they are thus deprived, they cannot have the use of the keys.
Reply to Objection 1. The matter of the sacrament of the Eucharist, on which the priest exercises his power, is not a man but wheaten bread, and in Baptism, the matter is simply a man. Wherefore, just as, were a heretic to be without wheaten bread, he could not consecrate, so neither can a prelate absolve if he be deprived of his authority, yet he can baptize and consecrate, albeit to his own damnation.
Reply to Objection 2. The assertion is true, provided matter be not lacking as it is in the case in point.
Reply to Objection 3. Sin, of itself, does not remove matter, as certain punishments do: so that punishment is a hindrance not because it is contrary to the effect, but for the reason stated.
Source: Summa Theologica (New Advent)