The set-up. Having identified original sin as an inversion of our maxims vs. their intended order of priority, Kant goes on to note that the nature of radical evil makes it empirically impossible to solve. If every particular choice we make is a reflection of our original choice, then a particular choice to reset that order would be conditioned by the inverse choice "in the beginning," and Kant seems to think that there is an incredible, but not absolutely knowably insurmountable, difficulty that we face in the process of redemption.
Another will than just our own. Kant does mention the "envoy from heaven" (Jesus Christ) and surmises that divine grace could, in an obscure and not overtly miraculous way, make up the difference between the particular redemptive maxims we try for, and the rigid order on the two elementary maxims. But in the Doctrine of Virtue he says:
Moral friendship, as contradistinguished from the æsthetical, is the entire confidence of two people, who reciprocally impart to one another their private opinions and emotions, so far as such surrender can consist with the reverence due from one to the other. ... pure moral friendship is no ideal, but is to be found extant here and there, in its perfection. [emphasis added]
A friend of man is he who takes an æsthetic participation in the welfare of his race, and who never will disturb it but with inward regret. This phrase, however, friend of man, is more limited than that of a philanthropist, for the friend cherishes the representation of the equality of his species, and has at least the idea of becoming indebted to them, even while he obliges them, where he figures to himself all mankind as brethren under a common Father, who wills their joint and common happiness.
Suppose, then, that when at least two people are friends, not only are their corrupted wills present and conjoint, but a third will, not necessarily God's or some savior's, is also present. If this hyperwill over the wills of the two friends is at least at first innocent enough that these two friends can, through this third will, choose to reset the order on the fundamental maxims of their souls, and by this means accomplish their initial redemption: for all that, is the Form of Friendship, so to say, something we can be friends with (allowing that Forms self-participate in some way, not like exemplars though), and in this unity of spirit transcend the Form of Evil inside us all?