Repentance means being cut down by the law’s declaration of judgment. It’s not an activity we do to prepare for grace, but a point of despair worked by God himself.
Luther's signature insight on the sacraments was that God’s word of promise doesn’t just symbolize an absent reality but that it gives and bestows God’s real favor.
Christmas is, therefore, the beginning of Christ’s earthly ministry, even while he awaits a number of years to gather his disciples and inaugurate his preaching of the kingdom.