John Cassian (ca. 360-435) was a world-travelling ascetic who thought a lot about suffering. And in his work, On the Death of the Saints (conference 6, chapter 11), he takes an aside to consider why God brings trials into our lives, both believers and unbelievers.1
Here’s my summary of Cassian’s reasons for suffering, along with a verse for each reason. He includes some of these verses and many others in the work I’m summarizing, which you can read online or in Accordance.
God uses suffering for probation.
And you shall remember the whole way that the LORD your God has led you these forty years in the wilderness, that he might humble you, testing you to know what was in your heart, whether you would keep his commandments or not. (Deut. 8:2 ESV)
God uses suffering to warn us.
When a scoffer is punished, the simple becomes wise; when a wise man is instructed, he gains knowledge. (Prov. 21:11 ESV)
God uses suffering for punishment and justice.
The soul who sins shall die. The son shall not suffer for the iniquity of the father, nor the father suffer for the iniquity of the son. The righteousness of the righteous shall be upon himself, and the wickedness of the wicked shall be upon himself. (Ezek. 18:20 ESV)
God uses suffering to reject us, which is “worse than all other punishments”
The bellows blow fiercely; the lead is consumed by the fire; in vain the refining goes on, for the wicked are not removed. Rejected silver they are called, for the LORD has rejected them. (Jer. 6:28-30 ESV)
God uses suffering to improve us.
We rejoice in our sufferings, knowing that suffering produces endurance, and endurance produces character, and character produces hope. (Rom. 5:3–4 ESV)
God uses suffering to prove his work in us.
You have been grieved by various trails, so that the tested genuine of your faith—more precious than gold that perishes though it is tested by fire—may be found to result in the praise and glory and honor at the revelation of Jesus Christ. (1 Peter 1:7 ESV)
God uses suffering to manifest his glory.
“It was not that this man sinned, or his parents, but that the works of God might be displayed in him.” (John 9:3 ESV)