Jesus says he will come unexpectedly
As it was in the days of Noah, so it will be at the coming of the Son of Man. 38For in the days before the flood, people were eating and drinking, marrying and giving in marriage, up to the day Noah entered the ark; 39and they knew nothing about what would happen until the flood came and took them all away. That is how it will be at the coming of the Son of Man. 40Two men will be in the field; one will be taken and the other left. 41Two women will be grinding with a hand mill; one will be taken and the other left.
42“Therefore keep watch, because you do not know on what day your Lord will come. 43But understand this: If the owner of the house had known at what time of night the thief was coming, he would have kept watch and would not have let his house be broken into. 44So you also must be ready, because the Son of Man will come at an hour when you do not expect him.
Peter heard this sermon with his own ears, and Jesus’ words must have made a lasting impression. The same comparisons appear in Peter’s description of the Last Day: it will be like the flood and it will come like a thief (see 2 Peter 3:6-10).
The difference between the flood and the Last Day is that God used water the first time, but he will use fire at the end. It is interesting that when John the Baptist contrasts himself with Jesus, he says, “I baptize you with water. But one more powerful than I will come, the thongs of whose sandals I am not worthy to untie. He will baptize you with the Holy Spirit and with fire” (Luke 3:16).
Jesus says the people who perished in the flood “knew nothing about what would happen until the flood came and took them all away.” But it certainly was not because they had not been warned. First of all, God gave them 120 years of grace between the time when he determined to send the flood and the day when it actually started raining (Genesis 6:3). Then the construction of the ark must have attracted some attention. People must have wondered why Noah was building such a big boat, especially when he did not live anywhere near a body of water that was big enough to float it. It was a natural conversation starter. We know that Noah was “a preacher of righteousness” (2 Peter 2:5). It is typical of God to warn people before he pours out his wrath upon them. It is typical of us to ignore the warning and be caught by surprise when his judgment descends. That is why, from our point of view, the Last Day will come like a thief.