Albert Barnes Commentary Psalms 103:11

"For as the heavens are high above the earth, So great is his lovingkindness toward them that fear him." — Psalms 103:11 (ASV)

For as the heaven is high above the earth — See the notes at (Psalms 57:10). Compare the notes at (Isaiah 55:9). The literal translation of the phrase here would be, “For like the height of the heavens above the earth.” The heavens—the starry heavens—are the highest objects of which we have any knowledge; and hence, the comparison is used to denote the great mercy of God—meaning that it is as great as can be conceived; that there is nothing beyond it; that we cannot imagine that it could be greater—as we can imagine nothing higher than the heavens.

So great is his mercy toward them that fear him — To those who reverence and serve him. That is, His mercy is thus great in forgiving their offenses; in imparting grace; in giving them support and consolation.