Now the birth of Jesus the Messiah took place in this way. When his mother Mary had been engaged to Joseph, but before they lived together, she was found to be with child from the Holy Spirit. Her husband Joseph, being a righteous man and unwilling to expose her to public disgrace, planned to dismiss her quietly. But just when he had resolved to do this, an angel of the Lord appeared to him in a dream and said, “Joseph, son of David, do not be afraid to take Mary as your wife, for the child conceived in her is from the Holy Spirit. She will bear a son, and you are to name him Jesus, for he will save his people from their sins.” All this took place to fulfill what had been spoken by the Lord through the prophet: “Look, the virgin shall conceive and bear a son, and they shall name him Emmanuel,” which means, “God is with us.” When Joseph awoke from sleep, he did as the angel of the Lord commanded him; he took her as his wife, but had no marital relations with her until she had borne a son; and he named him Jesus.
From the very beginning of the Incarnate Word's preparation to come among us we see the necessity of belief; palpable belief. First in the Virgin Mary: "Blessed is she who believed that there would be a fulfillment of what was spoken to her by the Lord" (Luke 1.45). Then Joseph, against all reason and his own plans believed what God spoke to him through an angel - and in a dream! If he - or Mary - had discussed the matter with his inner circle of friends they surely would have convinced him otherwise: that dreams cannot be obeyed over reason, that one must be sensible; God has given us minds for a reason and so on.
It seems the greatest acts of belief are reserved for the most secret inner recesses of the single individual's heart.