Mark Twain is quoted by Charles R. Swindoll as saying that we are all like a moon that has a dark side no one sees. This is true. We all have "feet of clay" and are vulnerable to sin because of our very nature. We cannot clean up our act before we can come to Jesus; we must come as we are, but we cannot stay that way.
We must see how bad we are before we can become good. It's not how bad we are, but how bad off we are. It is like the distance of a deaf man to a symphony or a blind man to the Mona Lisa. We cannot bridge the gap. Jesus sees through the veneer and we cannot fool him.
Humanists think mankind is basically good, but we an inherently bad. You must realize that we are not sinners because we sin, rather we sin because we are sinners. It is our constituted nature to sin. We can deal with sins in the plural, but our problem is sin in the singular--our old sin nature inherited from Adam. This is God's estimation of man, not man's estimation of man.
The totality of our nature is permeated with sin and our image of God is marred and defaced morally. "No one knows how bad he is until he has tried to be good," says C. S. Lewis as in a catch-22. The paradox is that we must see our bankruptcy--the truly bad person thinks he is all right! And Lewis adds, "We must realize how bad we are before we can be good." The way up, by paradox, is down.
We are sinful in toto and in solidarity with Adam completely. Someone has said, "We cannot escape our birthright." We cannot ingratiate ourselves with God, because we "have feet of clay." That means we have hidden vulnerabilities. We are permeated with sin through and through--there is no vestige of righteousness.
R. C. Sproul writes of a man who never lost his faith in the basic goodness of man despite being held captive in Iraq--this is sheer ignorance! Compared to Saddam Hussein the run-of-the-mill sinner looks like a saint; however, he is just as bad off from God's viewpoint and they both must come to Jesus the same way in childlike repentance and faith.
HOW DEPRAVED ARE WE THEN?
Man is depraved through and through, as bad off as he can be, but not as bad as he can be. N.B. that this is God's estimation of man, not man's estimation of man. It is like being pregnant; you can't be a little pregnant. Sin affects every aspect of our being which means we have radical corruption or total depravity. "For I know that nothing good dwells in me, that is in my flesh" (Rom. 7:18a). I don't mean utter depravity (that we are as bad as we can be) but total depravity (that every aspect of our being is spoiled with sin). We all have "feet of clay" (having flaws not readily apparent) and can't "clean up our act" nor ingratiate ourselves to God.
We show our solidarity in Adam when we sin--we sin because we are sinners, we are not sinners because we sin. It is not okay to fudge a little because we a diabolically alive--we all are like a moon that has a "dark side" no one can see. We may be a run-of-the-mill sinner compared to Hitler and see ourselves as saints in comparison; but Christ is the standard and exemplar, not Hitler.
We are inherently bad, biased to evil, having lost our inclination to good at the fall. Evil permeates our nature and we are defiant volitionally. This is all God's estimation of man, not man's estimation of himself. This is called original sin by some. Augustine of Hippo said we can only do evil (non-posse non-peccare). But Jesus sees through the veneer and facade. We are "by nature children of wrath" and "enemies" of God before we are saved. We must see how bad we are to be good and we don't know how bad we are till we've tried to be good.
There are many verses that support depravity including Jer. 17:9 ("The heart is deceitful above all things and desperately wicked, who can know it?"); Isa. 1:5-6, 64:6; Jer. 13:23; Rom. 8:8; Gen. 6:4-5. The law shows our present state: "Indeed, it is the straightedge of the law that shows us how crooked we are" ( Rom. 3:23, J. B. Phillips).
CAVEAT: EVEN OUR WILLS ARE STUBBORN AND DEPRAVED AND MUST BE WOOED AND TRANSFORMED BY GRACE AND THE HOLY SPIRIT! Soli Deo Gloria!