While people may indeed be leaving the "mainline" denominations, they are not becoming less "religious," and show every sign (especially among our younger population) of continuing to seek biblical answers to the bigger questions and issues of life.
While people may be moving toward pursuits of God that involve fewer of the traditional trappings, they are nonetheless finding answers in the traditional or proven truths -- transcendent truths centered on a transcendent God who, being bigger than our passing circumstances, and not just a product of them, is actually able to guide us.
Too often, instead of finding a church that is "salt" and "light," seekers find a compromised institution that, instead of offering a meaningful distinction from the world, merely reflects more of the same, along with a small "god" who moves with the flow of history rather than being sovereign over it. Most people recognize that this is not a god that anyone can find help from -- and it is not the God represented in the Bible.
A God who is big enough to be trusted is a God big enough to accomplish and communicate His will through His creation -- human writers included. While Jesus may not have said "write this down" (as earlier writers were specifically commanded to do), He did say that His Spirit would continue to equip us for an accurate handing-down of His teachings (John 14:25, 26).
Because man desires lordship over all that he views as "his," he will always resent and reject the idea of a holy God who lays claim to his life -- also rejecting any view of Scripture that supports that claim. The writers of the Bible were not afraid to proclaim this God, and His people today should not be, either.