What does Matthew 24:39 mean?

Jesus has told the disciples that nobody other than God the Father knows the day or hour of the second coming (Matthew 24:36). Now He is emphasizing that nobody will even see it coming. There will be no time for last-minute preparations. The signs of His return will be obvious to everyone—when they happen (Matthew 24:29–31)—but leading up to those moments, there will be no warning.

Christ has compared His return as judge and king (Revelation 19:11–15) to the onset of the flood of Noah's day. Moments before being wiped from the earth, people lived as they had always lived. They were eating, drinking, planning weddings, moving from one day to the next as if nothing would ever change. Realization came only in the last, too-late moments as the waters of the flood roared over them and swept them away.

Jesus does not mention the wickedness of the earth in the days of Noah (Genesis 6:5–8). Some believe that sin and evil on earth will mirror that of Noah's era. That's possible, but it's not something Christ explicitly predicts here. He only warns of the suddenness of the judgment of God. He has pictured His own return as something the nations will mourn (Matthew 24:30) and for which His people must live in a constant state of readiness (2 Peter 3:3–7; James 4:13–14).