Let's say the unbeliever decides to go on an "unbiased" investigation of the existence of God. In this investigation, he looks at the arguments from both sides and decides that God is real and His revelation is true. However, something interesting happened along the way. He went from viewing God as non-existent and non-authoritative to existent and authoritative. But how can this be? At some point, this unbeliever had to decide that God was the absolute authority over his reasoning process. That is, he had to agree that all knowledge is dependent upon God and His revelation (Proverbs 1:7); including the knowledge of unbelievers. And from this point forward, this former unbeliever will (if he wants to be consistent with Scripture) now present his message to unbelievers as follows: "Since God is the ultimate authority over all things, including man's reasoning, you must repent of your unbelief and give God the glory for your ability to reason; because without God, you couldn't reason at all."
When unbelievers ask for evidence for God, what "god" are they asking about? The biblical God? If so, then they are asking about a God who is the ultimate authority. That is, no evidence or human reasoning is higher than this God. According to the God of the Bible, He is the foundation of all knowledge and reasoning (Colossians 2:3). If the unbeliever admits that this is the God he is asking about, then he has to consider two options:
1. Believe in this God
2. Assert that such a God is impossibleMost unbelievers would be very hesitant to take option 2 because the burden of proof would be on them to substantiate their view. To assert that such a God can't exist would be to claim omniscience. Similarly, most unbelievers will not agree with option 1 either. The likely response from the unbeliever will be, "Why can't there be a third option; one that asserts that such a God must be proven by evidence and reason?"
If the unbeliever desires to go this route, he needs to understand a condition; he must believe that God's revelation is false from the outset. Why? Because this God is the ultimate authority and says things like, "he who is not with Me is against Me." (Matthew 12:30) Thus, if the unbeliever isn't in a position to accept that he is with Christ, then he is against Him even before his investigation. Bahnsen says it best:
Thus, the very thing that most unbelievers demand--a neutral and autonomous investigation of the facts to see whether God's word can be recognized as true and authoritative--already precludes the kind of God of which Scripture speaks. Apologists need to come to grips with this, lest their method of defending the faith betrays the object of their faith (Van Til's Apologetic, p. 97 fn. 23)Unbelievers must submit their reasoning to the God of Scripture. Otherwise, they are left in an epistemological bind whereby all knowledge ceases to be knowledge. Yet, the unbeliever will continue to "suppress the truth in unrighteousness" (Romans 1:18) and deny the God that he knows exists.