Psalms 103:8

This is a quotation, and knowing its source changes it entirely.

These words come from Exodus 34, where God passes in front of Moses and describes himself aloud. The setting matters: Israel had just made a golden calf while Moses was on the mountain receiving the law. The covenant was days old and already broken. That is the moment God chooses to state his own character, and this is what he says first.

So the sentence is not a general observation about divine kindness. It is the self-description God gave on the worst possible day, and it became the line Israel reached for afterwards — quoted by Nehemiah, Joel, Jonah, and here by David, whenever anyone needed to argue from who God is.

Slow to anger is literally long of nostrils in Hebrew, an idiom drawn from the flaring of the nose in rage. Long — the anger has a long fuse. It is not that there is no anger; the phrase assumes it. What is being described is the distance between provocation and response.

And abundant is a quantity word placed against a quality word. Mercy is not being described as sufficient or as available. There is a great deal of it, and the amount is offered as part of the character rather than as a policy that might be revised.