In that He takes our place it is decided what our place is.
In other words, we must understand humanity – and in particular the question of “fallen” and “unfallen” humanity – by looking at Christ, rather than interpreting Christ through abstract notions of “humanness” or “fallenness”. As Colwell writes (p.67):
Rather than arguing over whether or not Christ assumes “fallen” human nature (having defined the fall and fallenness in some other place and by some other means), we need to acknowledge that our fallenness is finally defined here (rather than somewhere else), in the flesh assumed by the Son and in the outcome of that assumption [i.e. the cross].
Or to put it another way: all anthropology is Christology. 🙂