Adam Clarke on Psalms 66:10

For thou, O God, hast proved us - This is a metaphor taken from melting and refining metals; afflictions and trials of various kinds are represented as a furnace where ore is melted, and a crucible where it is refined. And this metaphor is used especially to represent cases where there is doubt concerning the purity of the metal, the quantity of alloy, or even the nature or kind of metal subjected to the trial. So God is said to try the Israelites that he might know what was in them; and whether they would keep his testimonies: and then, according to the issue, his conduct towards them would appear to be founded on reason and justice.