As I see it, the numbers of people who those of us in the church would say are actually committed Christians—those who are practicing a vibrant faith—are not dying off. The Church is not dying. It is just being more clearly defined.
The "Nones" category is growing quickly, but the change is coming by way of Cultural and Congregational Christians who no longer feel the societal pressure to be "Christian." They feel comfortable freeing themselves from a label that was not true of them in the first place. Convictional Christians are not leaving the faith; the "squishy middle," as I like to call it, is simply being flattened.
So for those who really don't have any skin in the game, shedding the label makes sense.
As Christians find themselves more and more on the margins in American society, people are beginning to count the cost. While it used to serve Americans well to carry the label "Christian" in most circumstances (think about running for public office, for instance), it can actually be polarizing or considered intolerant now. So for those who really don't have any skin in the game, shedding the label makes sense.
As the trend continues, we will see the "Nones" continue to grow and the church lose more of its traditional cultural influence. Christians will likely lose the culture wars, leading to difficult times ahead for us. But we do not need to lose hope. This is not cause for despair. It is a time to regroup and re-engage.
Christianity may be losing its top-down political and cultural influence, but Jesus spoke of His followers making an impact in a very different manner. He taught that God's kingdom was subversive and underground. He used examples like yeast, which changes things from the inside, and mustard seeds, which are small and must be planted in order to grow up and out.
As the distinctions between Christians and an ever-growing post-Christian culture emerge, we will have to set aside any nominal belief systems and become active agents of God's Kingdom. The answer is not found in waging cultural wars incessantly, or in making a theological shift to the left to pacify a culture offended by the gospel. The answer is in all of God's people, changed by the power of the gospel and propelled by love, moving into the mission field as agents of gospel transformation.