Why God’s common grace is necessary for apologetics.

“When the case has been boldly made that the unbeliever must repent and admit to his suppressed knowledge of God if he is to have theoretical grounding for any knowledge whatsoever, then the Holy Spirit either opens his eyes at present or will do so at a coming day. In this present life God’s common grace restrains the apostate from utter intellectual self-destruction. While not positing neutral ground between the Christian and non-Christian interpretative systems, common grace nevertheless prevents the non-Christian from being consistent with his futile boast of autonomy. But in that coming day the unbeliever will cease to be schizophrenic and will fully become what he continually strived to be, but as yet could not be—completely independent of God. This will be hell.”
Bahnsen, Greg (2009-03-01). Presuppositional Apologetics: Stated & Defended (Kindle Locations 2808-2813). American Vision. Kindle Edition.

“When the believer argues with the unbeliever he should take this difference into account. The only reason there is not absolute antithesis in their use of language (and hence, hopeless confusion) is because of God’s common grace. Since the unbeliever is not completely epistemologically self-conscious, he can still hear and understand the word of the gospel. Yet in principle their respective outlooks and language would be absolutely divergent.”
Bahnsen, Greg (2009-03-01). Presuppositional Apologetics: Stated & Defended (Kindle Locations 2862-2865). American Vision. Kindle Edition.

“However, the unbeliever is still the image of God, living on metaphysical common ground with the Christian, and being restrained in his rebellion against God’s clear, necessary, sufficient, and authoritative revelation in nature and Scripture by common grace.”
Bahnsen, Greg (2009-03-01). Presuppositional Apologetics: Stated & Defended (Kindle Locations 2958-2960). American Vision. Kindle Edition.

A comical illustration of what apologetics would look like without God’s common grace enabling a point of contact between the believer and unbeliever:

https://www.youtube.com/v/kQFKtI6gn9Y?version=3&start=78&end=228&autoplay=1&hl=en_US&rel=0