One is more or less obvious: outright sinfulness, unrighteousness, lawlessness, self-indulgence, what the Bible would call "worldliness" or, perhaps in more modern dress, carelessness or heedlessness. In other words, we can just say to God, "No thanks, I don't want it, I'll take my own chances."
The other is much less obvious and more subtle, one that morally earnest people have much more trouble with: turning our back on the free gift and saying in effect, "I do agree with what you demand, but I don't want charity. That's too demeaning. So I prefer to do it myself. What you are offering is too cheap. I prefer the law to grace, thank-you very much. That seems safer to me."
What this means, of course, is that secretly we find doing it ourselves more flattering to our self-esteem–the current circumlocution for pride. The law, that is, even the law of God–"the most salutary doctrine of life"– is used as a defense against the gift. Thus, the more we "succeed", the worse off we actually are. The relationship to the giver of the free gift is broken…the Almighty God desires simply to be known as the giver of the gift of absolute grace. To this we say "no". Then the relationship is destroyed just as surely as it was by our immorality. To borrow the language of addiction, it is the addiction that destroys the relationship…One can be addicted either to what is base or to what is high, either to lawlessness or lawfulness. Theologically there is not any difference since both break the relationship to God, the giver.
The law is not a remedy for sin. It does not cure sin. St. Paul says it was given to make sin apparent, indeed to increase it. It doesn't do that necessarily by increasing immorality, although that can happen when rebellion or the power of suggestion leads us to do just what the law is against. But what the theologian of the cross sees clearly from the start is that, even more perversely, the law multiplies sin precisely through our morality, our misuse of the law and our "success" at it. It becomes a defense against the gift. That is the very essence of sin: refusing the gift and thereby setting what we do in the place of what God has done.
There is something in us that is always suspicious of or rebels against the gift. The defense that it is too cheap, easy, or morally dangerous is already the protest of the Old Adam and Eve who fear–rightly!–that their house is under radical attack. Since they are entrenched behind the very law of God as their last and most pious defense, the attack must indeed be radical. It is a battle to the death.
Gerhard Forde, On Being a Theologian of the Cross, pg. 26-28