But to be truthful, this was a misuse of Schaeffer's book title.
Superficially, it would make sense to take Schaeffer's title this way... 'In light of the conclusions to which this history of philosophy has led, how ought we to live?' But Schaeffer tells us (in the concluding Note) that the title comes directly from Ezekiel 33:10...
If our transgresions and our sins be upon us, and we pine away in them, how should we then live?In its larger context, this verse is addressing the fact that the Israelites of Ezekiel's day had turned away from acknowledging, knowing, and obeying the one true God. Not only that, but they despised those like Ezekiel sent by God to warn them of the consequences of acting as though He weren't necessary to their existence. The conclusion "How should we then Live?" means "Why would God allow us to continue to live if we carry on without any reference to him?"
Schaeffer is drawing the analogy between our times and those of Ezekiel. We have come to the point where our culture (like that of Europe when Schaeffer was writing) is naturalistic, denying the existence of God, of immaterial minds, and souls, and of the immortality of the human spirit. Even though many in our culture claim to believe in God, in practice we live and reason as though He were not our Creator and Sustainer. Like the Israelites of old, we must either repent of these attitudes and behaviors or be willing to accept it if God reveals His wrath upon us or gives us over to our own passions (Rom. 1:18-32).
Schaeffer's note ends,
This book is written in the hope that this generation may turn from the greatest of wickednesses, the placing of any created thing in the place of the Creator, and that this generation may get its feet out of the paths of death and may live.