Christianity is not a religion; it is the announcement of the end of religion. Religion consists of all the things (... behaving, worshiping, sacrificing) the human race has ever thought it had to do to get right with God. About those things, Christianity has only two comments to make. The first is that none of them ever had the least chance of doing the trick: the blood of bulls and goats can never take away sins (see the Epistle to the Hebrews) and no effort of ours to keep the law of God can ever succeed (see the Epistle to the Romans). The second is that everything religion tried (and failed) to do has been perfectly done, once and for all, by Jesus in his death and resurrection. For Christians, therefore, the entire religion shop has been closed, boarded up, and forgotten. The church is not in the religion business. It never has been and it never will be, in spite of all the ecclesiastical turkeys through two thousand years who have acted as if religion was their stock in trade. The church, instead, is in the Gospel-proclaiming business.
The Gospel of grace: salvation freely given out of love. Now comes the question: if we are sinners no matter how hard we try to be good, then why not just go ahead and do all sorts of bad, seemingly pleasurable activities?
The reason for not going out and sinning all you like is the same as the reason for not going out and putting your nose in a slicing machine: it's dumb, stupid, and no fun. Some individual sins may have pleasure still attached to them because of the residual goodness of the realities they are abusing: adultery can indeed be pleasant, and tying one on can amuse. But betrayal, jealousy, love grown cold, and the gray dawn of the morning after are nobody's idea of a good time.
On the other hand, there's no use belaboring that point, because it never stopped anybody. And neither did religion. The notion that people won't sin as long as you keep them well supplied with guilt and holy terror is a bit overblown. Giving the human race religious reasons for not sinning is about as useful as reading lectures to an elephant in rut. We have always, in the pinches, done what we damn pleased, and God has let us do it. His answer to sin is not to scream, "Stop that!" but to shut up once and for all on the subject in Jesus' death.
Furthermore, the usual objection... that people will take such graciousness on his part as permission to sin, is equally nonsensical. For one thing, he made us free, so we already have his permission—not his advice, mind you, nor his consent, nor his enthusiasm—but definitely his promise not to treat us like puppets. For another... the whole idea of people actually being encouraged to seduce maidens, or water stock, or poison wells by the agony and death of Jesus on the cross is simply ludicrous. We ourselves, thank you very much, are all the encouragement we need for dastardly deeds.
I am left, therefore, with the unhappy suspicion that people who are afraid the preaching of grace will encourage sin are in fact people who resent the righteousness they have forced themselves into. Having led "good" lives... they seethe inwardly at any suggestion that God may not be as hard on drug pushers and child molesters as they always thought he would be on themselves.
That explains things better than I could. Thanks, Robert F. Capon.