So great is his mercy toward. — Literally, Strong is his mercy upon (or, over). (Comp. Psalms 117:2.) The comparison in the first clause, and the use of this expression in Genesis 49:26 and 2 Samuel 11:23, suggests as the right rendering here
For as the heaven is higher than the earth,
So far (above what was expected) for them fearing him prevails his mercy.
(For the same comparison, see Isaiah 55:7-9; and comp. Isaiah 38:17; Micah 7:19.)