I thought this was an interesting quote from Pilgrims Progress (http://www.ccel.org/ccel/bunyan/pilgrim.iv.x.html
Christian: It may be very profitable; but do you begin.
Hopeful: Well, then, there are, in my judgment, four reasons for it:
1. Though the consciences of such men are awakened, yet their minds are not changed: therefore, when the power of guilt weareth away, that which provoked them to be religious ceaseth; wherefore they naturally turn to their own course again; even as we see the dog that is sick of what he hath eaten, so long as his sickness prevails, he vomits and casts up all; not that he doth this of a free mind, (if we may say a dog has a mind,) but because it troubleth his stomach: but now, when his sickness is over, and so his stomach eased, his desires being not at all alienated from his vomit, he turns him about, and licks up all; and so it is true which is written, The dog is turned to his own vomit again. 2 Pet. 2:22. Thus, I say, being hot for heaven, by virtue only of the sense and fear of the torments of hell, as their sense and fear of damnation chills and cools, so their desires for heaven and salvation cool also. So then it comes to pass, that when their guilt and fear is gone, their desires for heaven and happiness die, and they return to their course again.
2. Another reason is, they have slavish fears that do overmaster them: I speak now of the fears that they have of men; For the fear of man bringeth a snare. Prov. 29:25. So then, though they seem to be hot for heaven so long as the flames of hell are about their ears, yet, when that terror is a little over, they betake themselves to second thoughts, namely, that it is good to be wise and not to run (for they know not what) the hazard of losing all, or at least of bringing themselves into unavoidable and unnecessary troubles; and so they fall in with the world again.
3. The shame that attends religion lies also as a block in their way: they are proud and haughty, and religion in their eye is low and contemptible: therefore when they have lost their sense of hell and the wrath to come, they return again to their former course.
4. Guilt, and to meditate terror, are grievous to them; they like not to see their misery before they come into it; though perhaps the sight of at it first, if they loved that sight, might make them fly whither the righteous fly and are safe; but because they do, as I hinted before, even shun the thoughts of guilt and terror, therefore, when once they are rid of their awakenings about the terrors and wrath of God, they harden their hearts gladly, and choose such ways as will harden them more and more.
Christian: You are pretty near the business, for the bottom of all is for want of a change in their mind and will. And therefore they are but like the felon that standeth before the judge: he quakes and trembles, and seems to repent most heartily, but the bottom of all is the fear of the halter: not that he hath any detestation of the offence, as it is evident; because, let but this man have his liberty, and he will be a thief, and so a rogue still; whereas, if his mind was changed, he would be otherwise.
Hopeful: Now I have showed you the reason of their going back, do you show me the manner thereof.
Christian: So I will willingly.
1. They draw off their thoughts, all that they may, from the remembrance of God, death, and judgment to come.
2. Then they cast off by degrees private duties, as closet prayer, curbing their lusts, watching, sorrow for sin, and the like.
3. Then they shun the company of lively and warm Christians.
4. After that, they grow cold to public duty, as hearing, reading, godly conference, and the like.
5. They then begin to pick holes, as we say, in the coats of some of the godly, and that devilishly, that they may have a seeming color to throw religion (for the sake of some infirmities they have espied in them) behind their backs.
6. Then they begin to adhere to, and associate themselves with, carnal, loose, and wanton men.
7. Then they give way to carnal and wanton discourses in secret; and glad are they if they can see such things in any that are counted honest, that they may the more boldly do it through their example.
8. After this they begin to play with little sins openly.
9. And then, being hardened, they show themselves as they are. Thus, being launched again into the gulf of misery, unless a miracle of grace prevent it, they everlastingly perish in their own deceivings.