Summary

Portrait of Saint Anselm Saint Anselm Cur Deus Homo — Book Second, Chapter XXI

For, as man could not be reconciled but by the death of the God-man, by whose holiness the loss occasioned by man's sin should be made up; so fallen angels cannot be saved but by the death of a God-angel who by his holiness may repair the evil occasioned by the sins of his companions. And as man must not be restored by a man of a different race, though of the same nature, so no angel ought to be saved by any other angel, though all were of the same nature, for they are not like men, all of the same race. For all angels were not sprung from one, as all men were.
Source: Wikisource

Portrait of Saint Anselm Saint Anselm Cur Deus Homo — Book Second, Chapter XXI

And, therefore, if any one thinks that the redemption of our Lord ought to be extended even to the fallen angels, he is convinced by reason, for by reason he has been deceived. And I do not say this as if to deny that the virtue of his death far exceeds all the sins of men and angels, but because infallible reason rejects the reconciliation of the fallen angels.
Source: Wikisource

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