“sign.” Literally, Zechariah says, “according to what will I know it?” This is to be understood as asking for a sign. As Lenski writes, “it asks for a norm or sign in accord with which the promise will be fulfilled.” This is the same phrasing that Abraham uses in Genesis 15:8. Interestingly, scripture says that “Jews ask for signs” (1 Cor. 1:22), as was the case with Abraham, Gideon, and Hezekiah when they were promised things from the Lord. The difference with Zechariah was that he asked out of some measure of unbelief—as verse 20 makes clear—while these others asked from a desire to strengthen the faith they had (See Hendriksen). However, Zechariah’s unbelief can certainly be understood to some extent. The angel told him in terms that were clear to him that the Messiah that believers had been awaiting for some 4,000 years was about to come and his son would be the messenger and forerunner of the Messiah who had been prophesied about in the Old Testament. That could be hard to believe, even if the message did come from an angel. After all, the Bible had said the Messiah was coming soon in other places but it had been hundreds of years (cp. Joel 1:15; 2:1; 3:14; Isa. 13:6; 29:17-18; Zeph. 1:7, 14; Ezek. 30:3). Zechariah would no doubt have known those prophecies and realized that just because the Bible said the Day of Yahweh was near did not mean “near” in the sense of going to happen right away. Add to that, Zechariah knew it would take a miracle for his wife to give birth, and that would have contributed to his doubt in this situation. So he asked for a sign.