Greg Bahnsen has established that because God is the creator of all, all belongs to him. Because all belongs to him, either we live for him or we don't. Likewise with our intellects, we either think consistent with the reality that we are dependent upon God for all knowledge, or we think we are only dependent upon ourselves for knowledge and understanding.
That does not mean there is no point of contact or commonality with a professed unbeliever. The point of commonality is that we are God's creations, and we live in God's created world. Our common ground begins withe creation.
For there is something of great significance in common between the believer and unbeliever; they are both, irrespective of their saved and lost conditions, both the creaturely image of God. While the unregenerate needs to be renewed with respect to it, the image of God remains his. Men cannot cease being man, and to be man is to be God's image. Man is the finite replica of God, being like Him in every respect that is appropriate for the creature to resemble his Creator. Hereby no man can escape the face of God, for God's image is carried along with man wherever he goes - even into Hades. Therefore, the believer can find point of contact in his discussions with unbelievers deep down inside them. Creation establishes forever that no man is beyond the touch of God's revelation; men have been created with the capacity to understand recognized their Maker's voice.
This reality that we are all creations of God is actually quite crucial. This is where compassion begins; we recognize that those who are dead in their sins are still God's creatures. This is where "equality" among men and women begins, with our origins. Men and women aren't equal because it is legislated, or because it is "fair." It is because we are creatures of God.
It is not always easy to relate to an unbeliever. Over the weekend, I saw enough on Twitter from the entertainment industry to make me really loathe human beings. Yes, they mock God, perpetuate idolatry and exalt ugliness. But they bear the image of God. The only difference between them and me is that I have had the truth revealed to me, and have received the grace and forgiveness of God.