The distance between heaven and earth is unreachable.
David knew God’s mercy was much the same way.
Psalm 103:11 For as the heaven is high above the earth, so great is his mercy toward them that fear him.
The heights of the heavens could be used to describe the great heights of God’s mercy and grace toward us.
It was so vast and wonderful that it was beyond comprehension or description.
There’s a massive difference between a holy, gracious God and sinful men.
Although we would never be able to obtain any measure of His righteousness in ourselves, through His great mercy, God bridged the impossible divide!