What does Luke 12:40 mean?

The disciples are being taught to stay ready for Jesus' return after the ascension. This is a concept they won't understand until He leaves (John 14:26). Luke 12:1–34 and 12:41—13:9 give specifics for how to be ready. Jesus' followers stay ready in various ways. These include speaking only what builds up the kingdom of God. It means living with an integrity that does not fear death, rejecting the desire for wealth and the fear for meeting physical needs. It requires being responsible with the people and mission God has given, being willing to follow Jesus even if family doesn't. This readiness includes looking forward to Jesus' return, living in harmony with others, repenting, and showing long-suffering compassion toward others.

Luke 12:35–39 exhort Jesus' followers to stay ready, like servants awaiting their master's return from a wedding. They do not know when he will arrive—no more than a homeowner knows when a thief will break in. They must remain constantly vigilant.

Now, Jesus gives the context for His warnings: the Son of Man is coming, and the disciples and other Christ-followers don't know when.

The "Son of Man" is a prophetic title. It is taken from Daniel 7:13–14:

I saw in the night visions,
and behold, with the clouds of heaven
there came one like a son of man,
and he came to the Ancient of Days
and was presented before him.
And to him was given dominion
and glory and a kingdom,
that all peoples, nations, and languages
should serve him;
his dominion is an everlasting dominion,
which shall not pass away,
and his kingdom one
that shall not be destroyed.

At this time, the disciples don't understand that Jesus will die. They don't know He will be buried and come to life again. They certainly don't know that He will leave them and not return in their lifetimes. Moments before He ascends to heaven, they ask, "Lord, will you at this time restore the kingdom to Israel?" (Acts 1:6). But Jesus says, "It is not for you to know times or seasons that the Father has fixed by his own authority. But you will receive power when the Holy Spirit has come upon you, and you will be my witnesses in Jerusalem and in all Judea and Samaria, and to the end of the earth" (Acts 1:7–8).

Jesus makes several lessons on this theme for a reason. Even today, understanding the message is crucial: we do not know when Jesus will return. Interpretations of the exact timeline of the end times vary, within biblical possibilities. Details and applications are subject to debate. But Jesus is clear in Scripture that we do not know when He will return to take His followers away. There is no math, sign, or prophecy that gives us that date. We aren't meant to have the date. We are only meant to be faithful to the job He has given us (Matthew 28:18–20) and wait for Him.