Once, when he was serving before God in the order of his rank, by lot, as was usual with priests, it fell to him to enter the temple of the Lord for incense, and all the multitude of people prayed outside during the incense. 

Before God, only the pure perform the sacraments, but from the unclean God hides His face. "When did he come to cense?" Without a doubt, on the day of atonement, when one high priest entered the Holy of Holies, so that we might learn that just as this high priest, having entered the Holy of Holies, received fruit, so the Lord Jesus, the only and truly great Bishop, having entered the Holy of Holies, that is, with the flesh into heaven, received the fruit of His appearance in the flesh – our adoption as sons of God and salvation.

     Then an angel of the Lord appeared to him, standing on the right side of the altar of incense.

 An angel does not appear to everyone, but to a pure heart, as Zechariah was. The altar (in Church Slavonic - altar) is called incense because there was another altar - burnt offerings.

     And when Zechariah saw him, he was troubled, and fear came upon him. And the angel said to him, "Do not be afraid, Zacharias, for your prayer has been heard, and your wife Elizabeth will bear you a son, and you will call his name; John; and you will have joy and gladness, and many will rejoice at his birth.

 Zechariah is embarrassed; for the extraordinary vision disturbs even the saints. But the Angel stops the indignation. For everywhere for this reason Divine and demonic visions are recognized: if the thought is at first troubled, but then, with the dissipation of fear, soon calms down completely, then the vision is truly from God; but if fear and indignation are more intensified, then the vision is from demons. Why did the Angel say, "Thy prayer has been heard, and thy wife Elizabeth shall bear thee a son," since Zachariah prayed not for a son, but for the sins of the people? Some say: since Zechariah prayed for the sins of the people, but had to give birth to a son crying out: "Behold the Lamb of God, Who taketh away the sin of the world" (John 1:29), the Angel appropriately says to him: "Thy prayer for the remission of the sins of the people has been heard, for thou shalt bring forth a son, through whom shall be the remission of sins." Others understand it this way: Zechariah! Your prayer has been heard, and God has forgiven the people's sins. Then, as if he would say, "Whence can this be seen?" The angel said, "Behold, I give thee a sign: Elizabeth shall bear thee a son; and from the fact that Elizabeth will give birth, you must also be convinced of the remission of the sins of the people.

     For he shall be great in the sight of the Lord; he shall not drink wine or strong drink, and he shall be filled with the Holy Spirit from his mother's womb; and he shall 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 restore the hearts of the fathers to the children, and to the disobedient the minds of the righteous, that he may present to the Lord a prepared people.