John Calvin Commentary Genesis 7:12

"And the rain was upon the earth forty days and forty nights." — Genesis 7:12 (ASV)

And the rain was upon the earth. Although the Lord burst open the floodgates of the waters, He does not allow them to break forth all at once and immediately overwhelm the earth; instead, He causes the rain to continue for forty days. This was partly so that Noah, by long meditation, might more deeply fix in his memory what he had previously learned, by instruction, through the word, and partly so that the wicked, even before their death, might feel that the warnings they had derided were not empty threats.

For those who had so long scorned the patience of God deserved to feel themselves gradually perishing under His righteous judgment, which for a hundred years they had treated as a fable. And the Lord frequently so tempers His judgments that people may have leisure to consider more profitably those judgments that, by their sudden eruption, might overcome them with astonishment.

But the astonishing depravity of our nature reveals itself in this: if the anger of God is suddenly poured forth, we become stupefied and senseless; but if it advances at a measured pace, we become so accustomed to it that we despise it. This is because we do not willingly acknowledge the hand of God without miracles, and because we are easily hardened by a kind of acquired insensibility at the sight of God’s works.