Nicene and Post-Nicene Fathers, Vol. II:
On Christian Doctrine: Chapter 18

Chapter 18.—The Keys Given to the Church.

17.  He has given, therefore, the keys to His Church, that whatsoever it should bind on earth might be bound in heaven, and whatsoever it should loose on earth might be loosed in heaven; 1730 that is to say, that whosoever in the Church should not believe that his sins are remitted, they should not be remitted to him; but that whosoever should believe and should repent, and turn from his sins, should be saved by the same faith and repentance on the ground of which he is received into the bosom of the Church.  For he who does not believe that his sins can be pardoned, falls into despair, and becomes worse as if no greater good remained for him than to be evil, when he has ceased to have faith in the results of his own repentance.

Footnotes

Compare Matt. 16:19, Matt. 18:18.