Albert Barnes Commentary Isaiah 55:9

"For as the heavens are higher than the earth, so are my ways higher than your ways, and my thoughts than your thoughts." — Isaiah 55:9 (ASV)

For as the heavens ... - This verse is designed merely to illustrate the idea in the former. There is as great a difference between the plans of God and those of people, as between the heavens and the earth. A similar comparison occurs in Psalms 103:11—

For as the heaven is high above the earth,
So great is his mercy toward them that fear him.

Compare Psalms 57:10—

For thy mercy is great unto the heavens,
And thy truth unto the clouds.

Also Psalms 89:2—

Mercy shall be built up forever,
Thy faithfulness shalt thou establish in the very heavens.

The idea in all these passages is substantially the same—that the mercy and compassion of God are illimitable.