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 alcove 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.