Why are these two joined together – power and mercy – by Moses, as well as by Daniel, Nehemiah and Jeremiah? Why does he join strength, and greatness, and dreadfulness with mercy?
I say, to exalt mercy the more.
It sets forth and aggrandises mercy the more, that a God so great, so dreadful, should yet be merciful. The lion in Christ commends the lamb that is in him (Revelation 5:5-6) – that he that is so great, and strong, and terrible, should be a lamb.
Look – as the unworthiness and sinfulness of us, whom God loves and shows mercy to, commends his love (as you have it in Romans 5:8), so the greatness and terribleness of the person that loves, advances and magnifies his goodness and mercy, that he who is so great and terrible, and has such power, should yet be so merciful.
Mercy in the 1st verse of Psalm 89 meets in this God who, in the 7th and 8th verses, is so great a God, so fearful to all that are round about him. They who are nearest to him know him best: they say this of him – that this God is a God of mercy.
This begets a stupor, an amazement, that he who is able to rebuke all, and destroy all with a nod, should yet have so much love and mercy. This exalts and sets out his mercy, and makes it a wonder.
[This is my summary of Thomas Goodwin’s argument in Justifying Faith (Works, Vol 8, p48)]