The reason why the number of angels who fell must be made up from men.
Anselm. It was proper that God should design to make up for the number of angels that fell, from human nature which he created without sin.
Boso. This is a part of our belief, but still I should like to have some reason for it.
Anselm. You mistake me, for we intended to discuss only the incarnation of the Deity, and here you are bringing in other questions.
Boso. Be not angry with me; “for the Lord loveth a cheerful giver;” and no one shows better how cheerfully he gives what he promises, than he who gives more than he promises; therefore, tell me freely what I ask.
Anselm. There is no question that intelligent nature 211, which finds its happiness, both now and forever, in the contemplation of God, was foreseen by him in a certain reasonable and complete number, so that there would be an unfitness in its being either less or greater. For either God did not know in what number it was best to create rational beings, which is false; or, if he did know, then he appointed such a number as he perceived was most fitting. Wherefore, either the angels who fell were made so as to be within that number; or, since they were out of that number, they could not continue to exist, and so fell of necessity. But this last is an absurd idea.
Boso. The truth which you set forth is plain.
Anselm. Therefore, since they ought to be of that number, either their number should of necessity be made up, or else rational nature, which was foreseen as perfect in number, will remain incomplete. But this cannot be.
Boso. Doubtless, then, the number must be restored.
Anselm. But this restoration can only be made from human beings, since there is no other source.
Source: Cur Deus Homo (CCEL)