It was indeed unfortunate that our Christian "thinkers," in the time before the shift [from absolute truth] took place and the chasm was fixed, did not teach and preach with a clear grasp of presuppositions. Had they done this, they would not have been taken by surprise, and they could have helped young people to face their difficulties. The really foolish thing is that even now, years after the shift is complete, many Christians still do not know what is happening. And this is because they are still not being taught the importance of thinking in terms of presuppositions, especially concerning truth.-Francis Schaeffer; The God Who Is There Page 9
The flood-waters of secular thought and liberal theology overwhelmed the Church because the leaders did not understand the importance of combating a false set of presuppositions. They largely fought the battle on the wrong ground and so, instead of being ahead in both defense and communication, they lagged woefully behind. This was a real weakness which it is hard, even today, to rectify among evangelicals.
The use of classical apologetics before this shift took place was effective only because non-Christians were functioning, on the surface, on the same presuppositions, even if they had an inadequate base for them. In classical apologetics though, presuppositions were rarely analyzed, discussed or taken into account.
If this was true when Schaeffer wrote this in 1981, then how much more true is it today, with the postmodern shifts which have taken place! Given Schaeffer's analysis, it is more important today than ever before in history for us to have a firm grasp of presuppositions and their role in understanding truth. The presuppositional gulf between the Christian and the common pagan has never been wider than it is in our day.
And, if Schaeffer is right (which I think he is), the harder it will become to do classical apologetics. Classical apologetics requires something of a Scottish Common Sense epistemology, and yet - right or wrong - that old-hat approach to knowledge would hardly be acceptable to your average man on the street, or to your average educated person for that matter. As people move further away from the commonsense approach, the greater the challenge for the classical apologists who thinks that he can somehow prove God's existence by unbelieving standards without addressing the deeply flawed assumptions that your average nihilistic optimist brings to the table.