The first Sunday in Advent, according to Lutheran use, is noteworthy for its seemingly out of place Gospel, when it seems almost as if the deacon or priest has lost his place in the lectionary, and mistakenly turned to the reading for Palm Sunday, with St. Matthew's account of Christ's entrance into Jerusalem. The Feast of St. Thomas gives us a similar jolt from what we would normally expect in Advent, with a Gospel that seems to belong most properly in the Mass Quasi modo geniti, on Low Sunday, ie., the account in John 20 of Christ's post resurrection appearance to St. Thomas.
One of the things I love about this pericope is the pure faith that St. Thomas exhibits, when he beholds his resurrected Lord, and exclaims, "My Lord and my God." Dominus meus, et Deus meus. In the Holy Eucharist we, too, are blessed with a post resurrection appearance of our Lord Jesus. It is He, and not a phantasm, not a mere symbol. He showed Thomas His wounds, and just so, He comes to us in the Eucharist with His body that was torn and rent on the altar of Calvary, and with His blood that gushes out of his torn flesh, and washes over us. He bid Thomas to thrust his hand into His side, and just so, He bids us to find our sustenance and our life in the blood that comes forth from that side, precious blood that flows right from His sacred heart. Our Lord draws us to those wounds; in them we hide, that is, in them we find our identity. There we find our life. Like the pelican that sacrifices its flesh in order to feed her children, Christ nourishes and sustains us by pouring into us His own life, of which we are privileged to partake because we are baptized into His death.
After Christmas we are reminded right away, within its octave, that the Incarnation is a deadly serious matter, and not merely the celebration of a noble baby's birth, with feasts like those of Stephen, and the Holy Innocents. Likewise, we ought to let a feast like that of St. Thomas, just four days before Christmas, teach us that the advent of Christ among us will involve Him giving His all for us, and indeed, it involves us receiving all from Him, today and always, as often as we do this, in remembrance of Him.