Sermo 122, which does not have a clear date, is a complicated sermon, like so many others. In this sermon this has to do with a high level of allegory on all sorts of levels, where it is initially not so clear what Augustine aims at. We want to propose a narrative logic for the understanding of Sermo 122 that explains the argumentative flow of the sermon, the reasons behind its allegorical moves, the meaning of these moves, and the historical background of these moves. The thesis to be defended in this article is that the main interest in the sermon circles around a phrase from John that is only introduced at the very end of the sermon: "This is an Israelite, in whom there is no guile" (John 1:47-51). This phrase could be taken as linked to the key Pelagian claim in the Pelagian controversy, namely that a morally perfect human being is possible (at least in theory). Taking this passage at face value, it seems as if Jesus claims that Nathanael is morally perfect. What we suggest is that Augustine's main argumentative steps in the sermon are intended to reinterpret this passage in such a way as to postpone Nathanael's moral perfection to the afterlife, so that the possible Pelagian claim is effectively disarmed. Through a comparison with other places in Augustine where he discusses Nathanael (notably In Iohannis euangelium tractatus 7), we will make an attempt to situate this sermon in the context of the question of the continuity between Augustine's anti-Donatist doctrine of sin and his anti-Pelagian teachings on grace.