The unbeliever tells you, "I just can't believe there is a God. We don't need God to explain our existence. We can explain so much with science now anyway, and what we don't know it will eventually find out."
How do you respond to an objection like this?
Well, let's first examine what's not being said here. Everyone has a worldview - assumptions and presuppositions that we use to filter and interpret information from the world around us. As Christians, the fundamental assumptions of our worldview are that there is and must be a God, and that this God had a purpose in creation, and that He created us as rational beings for the purpose of communicating this purpose to us in such a way that it would be understood. We filter all our knowledge and information through these assumptions.
Our unbeliever does not share these assumptions. Rather, the highest authority in his universe is unaided human reason - that is the standard by which he judges things. In particular, he trusts that expression of human reason called "science" as his hope for explaining all things - including how we got here, where the universe comes from, etc.
Let's take a closer look at those assumptions, though. Are they consistent? Are they coherent with each other? Or is his worldview riddled with contradictions?
Science depends on two things to make it work.
1) Science assumes the fact that the universe is an orderly place, where things follow laws and behave in consistent and predictable ways. If a scientist wants to test a theory, he sets up a controlled set of circumstances and repeats an experiment several times. The assumption behind this method is that the universe is an orderly, consistent place so that if all things are equal, the same result should happen every time.
But - how do I know these laws of nature behave exactly the same way, every single time? I can't possibly drop every ball in the universe, under every possible circumstance, to verify that it will fall to the ground every time. But, if I
assume that the universe is fundamentally an orderly place, then it is a safe and reasonable thing to say that the ball will drop every time.
The unbeliever, though, has a problem. Science itself depends on this assumption - that the universe is fundamentally, at its most basic level, an orderly, law-abiding thing. But why should it be? If there is no intelligence or mind behind the universe, if there is no supernatural factor in play, and it's just a bunch of particles and energy bumping into each other, what reason to we have to believe that laws and principles, like, say, the laws of physics, are even possible?
In short, the unbeliever's assumption that there is no God is at odds with his assumption that the universe is (for some random reason?) an orderly, consistent, law-of-physics abiding place. That is an inconsistent worldview - the unbeliever is, in fact, "borrowing" the belief in the universe's orderliness from our worldview, where this belief actually makes sense!
2) Scientific research depends on honesty and accountability for results that can be trusted by society. But our unbelieving friend thinks there is no God. Where, then, does morality come from? If we are only the product of natural selection, and natural selection chooses those individuals who behave in a manner that improves their chances of reproduction, how do such values as honesty and truthfulness, self-examination and objectivity develop?
What if another Galileo appeared and proposed a similar, earth-shaking scientific theory that the scientific establishment didn't like? Galileo could have lost his life for his theories. Wouldn't natural selection have suggested a route
against the best interests of science - after all, giving in would have guaranteed his survival!
Our unbeliever's God-free, naturalistic assumptions leave him with no way to explain why scientists should be honest in their scientific research, if it is in their best interest to fudge their data. So how can he trust science in the first place - unless, as above, he's "borrowing" something from another worldview? A religious one, perhaps? But then he'd be inconsistent, wouldn't he?