How do you answer an unbeliever who rejects the Bible? The book of Proverbs provides us with a wise answer.
(Proverbs 26:4-5)"Answer not a fool according to his folly, lest you be like him yourself. Answer a fool according to his folly, lest he be wise in his own eyes." (ESV)
The unbeliever is called a fool because of his rejection of God, not because he is deficient as a person. These verses provide a two step process in answering the unbeliever.
In the first step we are told not to answer a fool according to his folly. It is often said that Christians cannot use the Bible when doing apologetics (defending the faith). This approach is actually answering a fool according to his folly.
The unbeliever is not operating on neutral ground because there is no neutral ground. He is assuming or presupposing a foundation of unbelief. One must have a starting point of belief or unbelief. The Christian must operate on the foundation of belief upholding the authority of God and his Word. The Bible is the ultimate and final authority, it is not dependent on any other authority for it's validation. Scripture is self attesting, there is no greater authority to which it can appeal.
In the second step we are told to answer a fool according to his folly. For the sake of argument we are to assume the worldview of the unbeliever and do an internal critique. All worldviews based on unbelief will ultimately end in contradiction, nonsense, and absurdity.
Only the Christian worldview provides the necessary preconditions for intelligibility. It is what must be in place in order for anything else to even be. Apart from the Biblical worldview nothing even makes sense.