John Calvin Commentary Genesis 9:1

"And God blessed Noah and his sons, and said unto them, Be fruitful, and multiply, and replenish the earth." — Genesis 9:1 (ASV)

And God blessed Noah. From this we infer with what great fear Noah had been downcast, because God, so often and at such length, takes pains to encourage him. For when Moses here says that God blessed Noah and his sons, he does not simply mean that the favor of fruitfulness was restored to them; but that, at the same time, God's design concerning the new restitution of the world was revealed to them.

For to the blessing itself is added the voice of God by which He addresses them. We know that brute animals produce offspring in no other way than by God's blessing; but Moses here notes a privilege that belongs only to men. Therefore, so that those four men and their wives, filled with apprehension, would not doubt for what purpose they had been delivered, the Lord prescribes to them their future condition of life: namely, that they were to raise up mankind from death to life.

Thus, He not only renews the world by the same word with which He previously created it, but He also directs His word to men. He does this so that they may recover the lawful use of marriage, may know that the care of producing offspring is pleasing to Him, and may have confidence that a progeny will spring from them. This progeny is to spread through all regions of the earth, making it inhabited again, even though it had been laid waste and made a desert.

Yet He did not permit promiscuous intercourse, but newly sanctioned that law of marriage which He had previously ordained. And although God's blessing is, in some way, extended to illicit connections, so that offspring is produced from them, yet this is an impure fruitfulness; that which is lawful flows only from God's expressly declared benediction.