I love this passage; like so many other good bits of Scripture it unpeels like an onion with layer after layer of meaning.
Zechariah is an old man, and let us allow him to be a good and decent one as well, for he is "righteous before God, living blamelessly." Still, old, and he and his wife Elizabeth are childless.
When the angel Gabriel appears to Zechariah, he is struck down by fear so that the angel first bids him, "Do not be afraid." If you think about it though, that's often the first thing angels say.... "Do not be afraid"..."Fear not"...I mean, we might be better off visualizing angels holding up this big sign "Don't Panic," rather than all this wings and flaming sword stuff. It seems to me, we take the wrong message about fear and terror when ordinary humans encounter the divine. It's not scary because God is wrathful and condemning and judgmental; God is simply not like that (mostly). It is terrifying because to directly encounter the divine is overwhelming. It is so much more than our human sense and human experience can handle that fear and terror are often the only way we can respond.
Gabriel lays a pretty heavy burden on kindly old Zechariah: "your prayers have been answered." Think about that: an angel of the Lord comes to you and tells you your prayers have been answered. Straight up. No doubt, no ambiguity, no searching for signs. Now you have to live up to that, try to be worthy of it. As if that weren't enough, Gabriel goes on to explain that their son will be "filled with the Holy Spirit...to make ready a people prepared for the Lord." So, yeah, no pressure there or anything.
Zechariah, though, asks a perfectly reasonably question, the one that, for me, is at the heart of this passage: "How will I know that this is so?"
Now as I said, Zechariah was a good man, but also an old man, and there is something that happens to you as you live long enough: You get to know things. Well, yes, but that's a GOOD thing, right? Right? Consider this: to know one thing is to not know or un-know other things. Learning things is often as much about closing doors - sorry, no unicorns or faster-than-light spaceships - as opening them. Zechariah was a kindly old man and he knew many things. He KNEW that angels do not appear standing next to you inside the temple sanctuary; and he KNEW that his wife and he were too old to have children; and he KNEW prophecies of children to become messengers of God just do not happen. Zechariah KNEW all those things, and yet they happened.
So Zechariah asked his perfectly reasonable question. Unfortunately for Zechariah, he was being relentlessly reasonable in the face of completely unreasonable events. The experience of God - divinity, what have you - isn't reasonable, in any sense of the word -- neither subject to merely rational comprehension, nor confined to polite boundaries. And so Gabriel, because the old man "did not believe my words", renders Zechariah mute until the prophesized events come to pass. I don't think Gabriel was punishing Zechariah; I think he was teaching him. Reason and intelligence are mighty gifts of God, yet there is an insidious trap for learned people to think they are tools fit for all purposes. Faith is not a subject to be studied, but a reality to be experienced. I suppose Gabriel could have explained all of this to Zechariah, had him re-read the good bits of Job if nothing else, but intellectually explaining the insufficiency of intellect is rather ironically pointless. Instead, Gabriel renders Zechariah mute, cutting him off, in some ways, from his rational/intellectual ways of "knowing" and leaves him to the experience of faith.