One of the biggest problems of moralistic, therapeutic deism (or MTD, on which I've written in the past) is its engulfing power to trivialize our sin. One of the key findings, if you read the book of Hebrews, is the danger of trivializing sin. Sin was a big deal in the garden, it was a big deal in the Old Testament, it was a big deal at the time of Christ, but in recent days it has been turned into anything other than a big deal. It has been called quite a few things less than it is, including a bad habit, an addiction, and even 'the way God made me'. I can't think of a more egregious bit of blasphemy, but it is a common bit these days.
The answer isn't necessarily found in the fundamentalist/liberal dichotomy (as if that were an accurate description in the first place). Fundamentalism reduces sin to certain behaviors. Sin is what those other people do. Liberalism reduces sin to social structures (what others aren't doing). Spiritualism reduces God to something inside us that simply needs enlightened, so that there is no such thing as sin, only 'inner darkness'.
Christian Smith (UNC, now at Notre Dame), came up with the term, MTD. He has now published a follow-up to his 2005 book, called 'Souls in Transition'. (Buy it here- http://fwd4.me/P0t ). It looks at where they (the subjects of the 2001-2005 study) are now. The statistics are fascinating, though too numerous to list here. For a good overview, watch/listen to Michael Horton's speech at this years Christless Christianity: 2010 West Coast Conference, called Moralistic & Therapeutic Deism. It can be found here-
This doctrine is the natural fallout of the idea that people are inherently good and the American corollary, "God helps those who help themselves." (A majority of evangelicals thought that was a verse from the bible in a recent Barna survey.) The problem is, it trivializes sin to the utmost. Failure to preach about sin is pastoral cruelty (C Fitzsimmons Allison). Eliminating sin by eliminating a holy God (which is what MTD does) is more than that, it is spiritual malpractice. One of Smith's findings was, the more a person attended an evangelical church or youth group, the more likely they were to embrace MTD. That's an ugly picture of what is happening in many of our churches and youth groups today.
When you hear sin depersonalized (it doesn't hurt anyone else, it's just a problem to your self-esteem), generalized (not a specific act, just a set of feelings or depression, etc.), and deflected to outsiders (those other people 'out there'), you know you are hearing MTD. 2 Tim. 3:2-5 is the biblical response to this kind of thinking. Flee.
[Note: I have not received any compensation for mentioning the book or the video above.]