Martin Luther on the oft-overlooked "third miracle" of the Annunciation:
Wherefore Saint Bernard declared there are here three miracles: that God and man should be joined in this Child; that a mother should remain a virgin; that Mary should have such faith as to believe that this mystery would be accomplished in her. The last is not the least of the three. The Virgin birth is a mere trifle for God; that God should become man is a greater miracle; but most amazing of all is it that this maiden should credit the announcement that she, rather than some other virgin, had been chosen to be the mother of God.
She did indeed inquire of the angel, "How can these things be?" and he answered, "Mary, you have asked too high a question for me, but the Holy Spirit will come upon you and the power of the Most High will overshadow you and you will not know yourself how it happens." Had she not believed, she could not have conceived. She held fast to the word of the angel because she had become a new creature. Even so must we be transformed and renewed in heart from day to day. Otherwise Christ is born in vain. This is the word of the prophet: "Unto us a child is born, unto us a son is given" (Isa. 9:6). This is for us the hardest point, not so much to believe that He is the son of the Virgin and God himself, as to believe that this Son of God is ours: That is where we wilt, but he who does feel it has become another man.
We who traffic in apologetics sometimes bristle at this reminder: that the single greatest obstacle to belief lies outside the purview of argumentation. Assertions that the Gospel narrative is too impossible or irrational or indemonstrable to be true are easily dispatched; that it is too good to be true is not.
Note Luke's juxtaposition of Mary's faithful "Amen" and Zechariah's unbelief in the face of likewise miraculous but--let's admit it--far less improbable news earlier in the chapter. Does this so-described "righteous" priest, who "walked blamelessly in all the commandments and statutes of the Lord" truly believe the almighty God of the universe incapable of opening a barren womb? How different might have been his reply had Gabriel's news been of some other elderly couple bearing a son! Good news is always more believable when it's for somebody else.
Tonight's O Antiphon speaks of Emmanuel: "God with us." But that God is with us is, of course, only good news if God is for us--or more to the point, for you. And indeed, if God is for you, who can be against you?