Must a universalist believe that Satan will be saved?
Not necessarily. A universalist could believe that God will save all humans but perhaps not fallen angels. We could call that 'human universalism'.
But it has been pointed out that the logic of the arguments that I employ in chapter 1 of my book would entail a more radical conclusion - that God will redeem all fallen creatures, both human and angelic. Let's call this 'radical universalism'. So the question then becomes, "Does Gregory MacDonald believe that Satan will be saved?"
I confess to being agnostic on the issue. Agnostic not because the logic of my arguments is not clear but because I am not sure what to think about the Devil.
Is the Devil a personal being? Suppose that he is (perhaps he is a fallen angel, although Scripture never spells out his origins). If Satan was once a good spiritual person who was later corrupted then the logic of my position is indeed that God could and would redeem him. There are two problems with this and both are big.
1. Scripture indicates otherwise (Rev 20)
2. the Christian tradition is clear that Satan will not be redeemed (indeed, contrary to popular opinion, not even Origen claimed that Satan would be saved).
The only way that I can think to get out of this bind is to suggest that Lucifer (the good being created by God) will be saved but that Satan (the name for Lucifer-as-corrupted) will be destroyed. This is analogous to the way that God destroys our old natures in Christ and makes us new creations. If anyone is in Christ - new creation! Old things have passed away and all things have become new. Satan is dead. Long live Lucifer. But, I freely confess, this is very speculative and it is not what I actually believe.
Suppose then that Satan is not actually a personal being at all. Suppose that he is a personification of evil or, more plausibly, some kind of epiphenomena supervening on human evil (individual and social). I am thinking along the lines of Walter Wink here. On this view of Satan then even a radical universalist such as myself would insist that Satan cannot be saved. Indeed, quite the opposite! Satan must be purged from the created order in order for there to be radical, cosmic redemption.
If you have thoughts on this difficult topic please do post a comment.