John Calvin 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)

And knew not until the deluge came. The source and cause of their ignorance was that unbelief had blinded their minds; on the other hand, we are informed by the Apostle that Noah, by the eyes of faith, perceived at a distance the vengeance of God which was still concealed, so as to dread it in advance (Hebrews 11:7). And here Christ compares Noah with the rest of the world, and Lot with the inhabitants of Sodom, so that believers may learn to withdraw, so that they do not wander and are not cut off along with others.

But it must be observed that the reprobate at that time were hardened in their wickedness, because the Lord did not show His grace to any but His servants, by giving them a salutary warning to beware in proper time. Not that information of the future deluge was altogether withheld from the inhabitants of the world—before whose eyes Noah, in building the ark for more than a hundred years, presented a warning of the approaching calamity—but because one man was specially warned by divine revelation of the future destruction of the whole world and was raised up to cherish the hope of salvation.

Though the report of the last judgment is now widely circulated, and though there are a few persons who have been taught by God to perceive that Christ will come as a Judge in due time, yet it is proper that these persons should be aroused by this extraordinary kindness of God. Their senses should be sharpened, so that they do not give themselves up to the indifference that so generally prevails. For Peter compares the ark of Noah with our baptism on this ground: that a small company of men, separated from the multitude, is saved amidst the waters (1 Peter 3:20–21). To this small number, therefore, our minds must be directed if we desire to escape in safety.