Aquinas asks: Whether the will of the demons is obstinate in evil? (Summa Theologica 1.64.2). He considers several arguments in favour:
- That demons have intellect and could therefore choose to be good again.
- That God's infinite mercy exceeds the finite malice of the demons.
- That the demons' chief sin was pride, but they no longer have anything to be proud about.
- Human sin is pardonable.
- The demons are not wholly evil, since they are capable of stating divine truths (example: "You [Jesus] are the holy one of God", Mark 1:24).
But he comes down against the proposition, quoting Psalm 73:23 (a.k.a 74:23)
The pride of them that hate thee ascendeth continually
and also refers to the Biblical assertions of everlasting punishment for "the devil and his angels". The five arguments are rejected as follows:
- They do not have free will, as we do.
- God's pardon requires that they repent, but they will not.
- The devil still has the will or desire to replace God, even though he does not have the power (and never did).
- The reasons why human sin can be pardoned are different.
- Even if the devil does something apparently "good", such as quoting Scripture, he is doing it to serve an evil purpose.
Elsewhere in 1.50-64 he discusses the nature of angelic/demonic intellect and will. For the question of whether they can change sides, his answer is no. The reason is that the angels do not learn and grow as we do; all their knowledge and inclinations are fixed from the moment of their creation, and they cannot change that. Good angels will remain good and bad ones (demons) will remain bad. He says that Satan must have fallen immediately after his creation (1.63.6), causing all the other devils to fall in the same instant (1.63.8), and that the remaining good angels have been made blessed by their meritorious deeds and therefore are blessed forever, incapable of sinning (1.62.8).
In the bigger picture, the reasons why humans can be saved are bound up in the nature of who we are - made in God's image, sinners but beloved, and redeemed through God becoming incarnate as one of us. That is not true of fallen angels.