A question that Christian children sometimes (and Christian adults too seldom) ask is whether, if they should pray for their enemies, that means that they should pray for Satan – i.e., pray for Satan’s eventual reformation and redemption.
The traditional answer is that angels’ mode of existence, between time and eternity, is such that their choices do not take place in successive sequence but rather permeate their entire existence and so are irrevocable; hence an angel who chooses sin has no possibility of redemption, meaning that praying for Satan would be pointless.
But what exactly is this mode (“aeveternity”) intermediate between time and eternity supposed to be? Aquinas reviews several accounts of aeveternity and proposes his own. The problem is that, perhaps apart from one option that Aquinas dismisses as incoherent, none of the accounts seems incompatible with angels’ choices being revocable. In any case, if one can petition God, whose choices are supposed to be timeless – and if Catholics petition saints, whose choices are supposed to be aeviternal – then apparently trying to influence the choices of nontemporal agents is kosher, so why should Satan’s nontemporal character be a bar to hoping for him to mend his ways?
This is a purely hypothetical debate for me, since I don’t believe in Satan (nor in the coherence of a timeless agent’s interacting with temporal events, for that matter), but it’s interesting nonetheless. (I implicitly took a side in a story I wrote in high school.)