When we argue presuppositionally we look for a point of contact. What does that mean?

Every person has an inescapable sense of deity.

Romans 1:18-23 The wrath of God is being revealed from heaven against all the godlessness and wickedness of men who suppress the truth by their wickedness, 19since what may be known about God is plain to them, because God has made it plain to them. For since the creation of the world God’s invisible qualities—his eternal power and divine nature—have been clearly seen, being understood from what has been made, so that men are without excuse. For although they knew God, they neither glorified him as God nor gave thanks to him, but their thinking became futile and their foolish hearts were darkened. Although they claimed to be wise, they became fools and exchanged the glory of the immortal God for images made to look like mortal man and birds and animals and reptiles.

We all know God in one way or another, but we choose not to acknowledge Him or honor Him. We process the data incorrectly and instead of acknowledging God’s invisible qualities we try to suppress the truth.

But the amazing thing is that we live in God’s world. Every place in creation, in the universe, on this earth… any place is a starting point where God reveals himself. This is not only common ground, it is God’s common grace. We cannot build a philosophy upward to God. When we talk to non-Christians, they are God conscious, but not God-acknowledgers. For the gospel must be sought by the natural man and at some level he is a covenant breaker.

To put it another way, metaphysically we have everything in common but epistemologically we have nothing in common. We are image bearers of God. We are part of the creation. The place of difference is our ultimate religious commitments. Foundational level we have nothing in common with unbelievers. So when you talk with an unbeliever (and even a believer) you need to challenge their foundations and heart commitments.