Jesus states that no one, not even the angels, knows the day or hour of his return, and that it will be a sudden and surprising event like a thief in the night or the flood of Noah. The passage emphasizes that believers must "be ready" and "watch," because his coming will be unexpected. The illustrations of one person being taken and another left from a pair in the field or at the mill highlight the sudden and unexpected nature of his arrival, urging constant vigilance.