Faith is the Engine
Zechariah was taking his turn in the Temple burning incense when Gabriel approached him. Zechariah and his wife, Elizabeth, were both "walking blamelessly in all the commandments and statutes of God," (Lk 1:6) and were considered by God to be "righteous." Elizabeth was barren and getting on in years. While Zechariah is burning incense, the "whole multitude of the people were praying outside." (v.10) It's a pretty powerful time.
Then Gabriel shows up. So, imagine -- in the temple alone; it's quiet with candles flickering and glinting off the gold; incense burning; everybody outside praying; Zechariah and Elizabeth have been praying for a child; the angel shows up -- this is powerful!
"And Zechariah was troubled when he saw him, and fear fell upon him. But the angel said to him, "Do not be afraid, Zechariah, for your prayer has been heard, and your wife Elizabeth will bear you a son, and you shall call his name John. And you will have joy and gladness, and many will rejoice at his birth, for he will be great before the Lord. And he must not drink wine or strong drink, and he will be filled with the Holy Spirit, even from his mother's womb. And he will turn many of the children of Israel to the Lord their God, and he will go before him in the spirit and power of Elijah, to turn the hearts of the fathers to the children, and the disobedient to the wisdom of the just, to make ready for the Lord a people prepared." And Zechariah said to the angel, "How shall I know this? For I am an old man, and my wife is advanced in years."" (Luke 1:12-18 ESV)
So Zechariah gets this great prophetic "Word" from Gabriel himself, and he responds by basically asking for another sign or guarantee. "How shall I know this?" I don't believe it yet, so give me another sign or guarantee, etc. so my mind will believe. He did not believe the angel! This of course brings a reaction from Gabriel, who makes Zechariah mute until the baby is born. Without faith, we constantly seek a greater surety.
Now to Mary. She is a virgin betrothed to Joseph. Mary is not in a worship service; the multitude is not outside praying; she is not burning incense. She is just going about her life and the angel shows up, and makes an outlandish promise -- "And the angel said to her, "Do not be afraid, Mary, for you have found favor with God. And behold, you will conceive in your womb and bear a son, and you shall call his name Jesus. He will be great and will be called the Son of the Most High. And the Lord God will give to him the throne of his father David, and he will reign over the house of Jacob forever, and of his kingdom there will be no end." And Mary said to the angel, "How will this be, since I am a virgin?"" (Luke 1:30-34 ESV)
Mary is not asking for a great assurance, or additional reasons to have faith. Her question assumes that the promise is going to come to pass, she just doesn't know how. And then Gabriel explains it to her, and ends up saying, "For nothing will be impossible with God."" (Luke 1:37 ESV) Later Mary visits Elizabeth, and Elizabeth by the Spirit says, "And blessed is she who believed that there would be a fulfillment of what was spoken to her from the Lord."" (Luke 1:45 ESV) She is speaking by the Holy Spirit, Who gives God's own commentary on Mary, that she believed the Word spoken over her would come to pass. Zechariah, on the other hand, is made mute and unable to testify in faith until after his promised son is born.
Zechariah had everything maximized for him:
Big worship service
Incense in the temple;
He and Elizabeth had been praying for a child;
The multitude outside praying
He and Elizabeth had been commended by God;
Pregnancy would take away the reproach of barrenness (Lk 1:25)
Mary had a different circumstance:
Mary took responsibility for her own life and her own faith and did not wait for a greater assurance or a better circumstance. "Now faith is the assurance of things hoped for, the conviction of things not seen." (Hebrews 11:1 ESV) Faith is the engine that drives the Kingdom. Believe God!