What does Luke 21:29 mean?

It is the week before the crucifixion. Jesus spends His nights on the Mount of Olives but His days in the temple Court. There, He instructs the people and debates religious leaders (Luke 21:37–38). He has just defended His authority before a crowd, humiliating and angering the Pharisees, elders, scribes, and Sadducees (Luke 20). As Jesus and the Twelve return to the Mount of Olives, the disciples marvel at the beauty of the temple. Jesus gives them an ominous warning: it will not stand forever; it will be destroyed. Peter, Andrew, James, and John ask when (Mark 13:3; Luke 21:5–7).

In response, Jesus describes the hardships the world—but especially the Jews—will face between His ascension and second coming. The temple will be destroyed "when [they] see Jerusalem surrounded by armies" (Luke 21:20). This happened in AD 70 when the Romans sacked Jerusalem. But Jesus also predicts persecution, war, murder, and natural disasters (Luke 21:8–24). Now, He wants them to understand a much more important event: when He returns to judge the world (Luke 21:25–28).

Jesus has said, "But concerning that day or that hour, no one knows, not even the angels in heaven, nor the Son, but only the Father" (Mark 13:32). Later, shortly before Jesus returns to heaven in a cloud of glory, the disciples will ask Him, "Lord, will you at this time restore the kingdom to Israel?" Jesus responds, "It is not for you to know times or seasons that the Father has fixed by his own authority" (Acts 1:6–7). We don't know if Jesus knows the date of His return now—at this moment in the modern world—but it's clear that we, fallible people, do not.

That doesn't mean there won't be signs when the time draws closer. The events of the tribulation will be as clear as the leaves on a fig tree that show summer is coming (Luke 21:29–30). At the end, Jesus will return.