What does Luke 1:18 mean?

Zechariah and his wife, Elizabeth, are getting older and have never had children. Even—or especially—for godly, righteous people, this would have been a difficult experience (Luke 1:5–7). Zechariah is a priest, chosen to offer incense in the temple on a particular day (Luke 1:8–10). While alone in the room containing the altar, he has seen an angel, who predicts that the couple's request will be answered: Elizabeth will have a son (Luke 1:11–13). Further, that son will play an important role in fulfilling prophecy (Luke 1:14–17; Malachi 3:1; 4:5–6).

It would be natural to wonder at such a miraculous statement, even in the presence of an angel. There would be many questions about how such a thing would happen. Wondering "in what way" is how Mary, given even more miraculous news, will respond (Luke 1:34). Zechariah's response here seems more about "if," rather than "how." The NASB clarifies this by including the phrase "how can I know this for certain?"

It's that tinge of doubt—doubt that this event really will occur—which earns Zechariah a relatively mild rebuke from the angel (Luke 1:19–20).