Jesus makes a comparison between how it was in Noah’s day with how it will be when Christ returns. In the days before the flood, Noah knew what was coming. For 120 years He was preaching (2 Peter 2:5) and preparing (Hebrews 11:7), but people continued with their normal activities. Despite Noah’s warnings, they remained unaware or unconvinced that destruction was coming upon them. The crisis was unexpected. The unbelievers were not prepared for God’s judgment. Only when it was too late did the people know how wrong they were.
They were eating and drinking, marrying and giving in marriage. Jesus had referred to this imagery before (Luke 17:26-27). The point is not that these things were wicked in themselves, but that people are so absorbed in worldly pursuits that they pay no attention to solemn warnings. Some were planning for their future on the earth, but not for their future in eternity. Jesus wanted His disciples to understand the signs of the times, and be ready, unlike those who did not believe and would not understand, and not be ready.
One will be taken and one left. This does not refer to believers being taken out of the trouble, but rather to unbelievers who are taken in judgment during the trouble. Just as people were taken away in judgment by the flood of Noah’s day, so too, will people be taken away in judgment during the return of Jesus.
One of the many things that separate believers from unbelievers is that believers trust God’s prophetic word. An unbeliever has to be shown these things through experience. If you have to learn this lesson by experience, it will be too late. The time to get aboard the Ark is now.