Matthew 24:40

At that time there will be two men [working] in a field; one will be taken [i.e., to be with the Lord. See I Thess. 4:17], while the other one will be left [i.e., to be punished by the Lord. See II Thess. 1:7-9].

Matthew 24:50-51

that slave's master will return on a day that he least expects, and at an unknown hour.

1 Thessalonians 5:2-3

for you know perfectly well that the day of the Lord [i.e., when He returns, See II Thess. 2:1-2] will arrive like a thief coming in the night.

Treasury of Scripture Knowledge

Summary

But God shall shoot at them with an arrow; suddenly shall they be wounded.

Bible References

Suddenly

Matthew 24:40
At that time there will be two men [working] in a field; one will be taken [i.e., to be with the Lord. See I Thess. 4:17], while the other one will be left [i.e., to be punished by the Lord. See II Thess. 1:7-9].
1 Thessalonians 5:2
for you know perfectly well that the day of the Lord [i.e., when He returns, See II Thess. 2:1-2] will arrive like a thief coming in the night.