This reduction of the faith is one of the things David Wells decries in his book, The Courage to Be Protestant. He does so while explaining both the nature and the demise of classical evangelicalism.
The first constituency...is classical evangelicalism. This is what took shape and form immediately following the Second World War, both in Europe and in the United States. What stood out about it, and what still does, is its doctrinal seriousness (p. 2)Wells explains that there were two core theological beliefs that held an otherwise theologically diverse group together: (1) the full authority of the inspired Scripture and (2) the necessity and centrality of Christ's penal substitution.
What this meant for them was that faith that was biblical would, of necessity, be doctrinal in its form. This, in fact, was so much more than simply asserting the inspiration of Scripture and its inerrancy. In the early days of the movement, a whole way of thinking grew out of this primary commitment. It meant that being biblical in tone and content was central. From this grew churches that valued biblical truth and Christian life that sought its nourishment in the Word of God (p. 5, italics added)A biblically informed view of life. So far, so good. So what happened? How did the evangelical world get into the mess it's in today? I think Wells' analysis is spot on. A worthy goal of uniting around the essential doctrines of the faith degenerated into reducing the faith to the essentials.
Through the 1950s, 1960s, and even 1970s, much else besides the two core principles was part and parcel of evangelical belief and practice. There was, however, a tacit agreement that liberty would be allowed in all these other matters provided that the core principles were honored... What happened, though was that this doctrinal vision began to contract. The goal that diversity in secondary matters would be welcomed quite soon passed over into an attitude that evangelicalism could in fact be reduced simply to its core principles of Scripture and Christ. In hindsight, it is now rather clear that the toleration of diversity slowly became an indifference toward much of the fabric of belief that makes up Christian faith...
The capacity to think doctrinally was bieng lost as new leaders emerged, as the leadership of the evangelical world shifted from the older pastor-theologians to the new entrepeneurial organization builders, and as churches began to reflect this change in their attitudes and worship.
The erosion in biblical ways of thinking at first passed almost unnoticed. Nevertheless, after a while it was hard to miss the fact that this was happening. No doubht there were many specific causes, Campus organizations were undoubtedly reducing Christian faith to its most minimal form. And as seious biblical preaching in the churches diminished, ignorance of biblical truth became commonplace. But the largest factor in this internal change, I think, was that evangelicalism began to be infested by the culture in which it was living. And then Christianity became increasingly reduced simply to private, internal, therapeutic experience.