Who is the doctrine of depravity for? As in, what good does it do?
For a few reasons I would argue that considering man in his sinful state starts as good for believers, and not only so that we know what we’re up against while doing evangelism.
Take Titus 3:3. That verse describes our foolishness, disobedience, slavery to passions and pleasures, hating and being hated, and it begins with a “for.” The argument from the “for we ourselves were once” bases every Christian’s motivation to do good to, and be patient with, those still stuck passing their days in malice and envy.
Take Ephesians 2:1-3. This is the setup for perhaps the greatest adversative conjunction in all the inspired text: “But God, being rich in mercy….” in verse 4. Before that, the picture is of our spiritual deadness in trespasses, being sons of disobedience, slaves to the desires of the body, and by nature children of wrath. And yet the apostle doesn’t frame depravity as a target for the gospel, he frames it as a basis for our praise of God’s glorious grace.
You, Christian, were dead. You once walked in sins. We all once lived in the passions of our flesh. This is true for all mankind. But this doctrine teaches that “even when we were dead in our trespasses, [God] made us alive together with Christ”!
So believers don’t forget their once, and they definitely don’t have Depravity-of-Man beat-sticks for whacking unbelievers. They have reasons to worship humbly and remember God’s immeasurable riches of grace in kindness. That will be witness to those who are perishing.