Charles Ellicott Commentary Matthew 24:39

"and they knew not until the flood came, and took them all away; so shall be the coming of the Son of man." — Matthew 24:39 (ASV)

So shall also the coming of the Son of man be — These words justify the interpretation given above of Matthew 24:29-30. If the “signs” of the Advent were to be phenomena visible to the senses, there could not be this reckless apathy born of ignorance. If they are to be tokens, “signs of the times,” which can be discerned only by the illumined insight of the faithful, the hardened unbelief on one side, and the expectant watchfulness on the other, are the natural result of the power or the lack of power to discern them.