And when Zacharias saw him, he was troubled, and fear fell upon him.

Wesley's Notes

1:12 Zacharias was troubled - Although he was accustomed to converse with God, yet we see he was thrown into a great consternation, at the appearance of his angelical messenger, nature not being able to sustain the sight. Is it not then an instance of the goodness is well as of the wisdom of God, that the services, which these heavenly spirits render us, are generally invisible?

Matthew Henry's Concise Commentary

1:5-25 The father and mother of John the Baptist were sinners as all are, and were justified and saved in the same way as others; but they were eminent for piety and integrity. They had no children, and it could not be expected that Elisabeth should have any in her old age. While Zacharias was burning incense in the temple, the whole multitude of the people were praying without. All the prayers we offer up to God, are acceptable and successful only by Christ's intercession in the temple of God above. We cannot expect an interest therein if we do not pray, and pray with our spirits, and are not earnest in prayer. Nor can we expect that the best of our prayers should gain acceptance, and bring an answer of peace, but through the mediation of Christ, who ever lives, making intercession. The prayers Zacharias often made, received an answer of peace. Prayers of faith are filed in heaven, and are not forgotten. Prayers made when we were young and entering into the world, may be answered when we are old and going out of the world. Mercies are doubly sweet that are given in answer to prayer. Zacharias shall have a son in his old age, who shall be instrumental in the conversion of many souls to God, and preparing them to receive the gospel of Christ. He shall go before Him with courage, zeal, holiness, and a mind dead to earthly interests and pleasures. The disobedient and rebellious would be brought back to the wisdom of their righteous forefathers, or rather, brought to attend to the wisdom of that Just One who was coming among them. Zacharias heard all that the angel said; but his unbelief spake. In striking him dumb, God dealt justly with him, because he had objected against God's word. We may admire the patience of God towards us. God dealt kindly with him, for thus he prevented his speaking any more distrustful, unbelieving words. Thus also God confirmed his faith. If by the rebukes we are under for our sin, we are brought to give the more credit to the word of God, we have no reason to complain. Even real believers are apt to dishonour God by unbelief; and their mouths are stopped in silence and confusion, when otherwise they would have been praising God with joy and gratitude. In God's gracious dealings with us we ought to observe his gracious regards to us. He has looked on us with compassion and favour, and therefore has thus dealt with us.