God exists.

My opponent presents the following two reasons to think that knowledge depends upon God:1. Plantinga's proper functionalism is the correct account of knowledge, and it requires the existence of God for knowledge to exist.2. For anyone to have knowledge, there has to be an omniscient being.Both of these arguments take the following form:1. If God did not exist, knowledge would not be possible.2. Knowledge is possible.3. Therefore, God exists.Preliminary NotesFirst, note that an argument of this form implies a form of skepticism in the absence of an independent reason to believe in the existence of God. Because if the first premise is true, then any doubt about the existence of God turns into doubt about whether we have knowledge. Premise 1 therefore leads to a form of skepticism - doubt about whether we can "know that we know" anything. I'll elaborate on this in connection with each specific argument.Second, note that this argument does not establish the existence of God as defined in round 1: an omnipotent, omniscient, all good person. Nothing in the argument from proper functionalism implies that God has to be omnipotent, omniscient, or all good - he could just be a finite tinkerer who goes around creating rational agents throughout the universe with properly functioning cognitive faculties. And the second argument, from omniscience, only establishes at most that attribute, not omnipotence or omnibenevolence.Third, note that God's existence is arguably incompatible with the existence of knowledge, not necessary for its existence. If the universe operates naturalistically, then we can be assured that our cognitive faculties and the world around us will always operate consistently, in accordance with the laws of nature. However, if God exists, then we can never be sure of this, because there is no way of knowing whether he will choose to deceive us or alter the course of nature at any given time. He could even miraculously interfere with our brains to make the most egregiously erroneous propositions appears self evident to us.Fourth, note that the argument would apply just as much to God as it would to us. If we need to be created by a higher being to be assured that our cognitive faculties are reliable, then God also needs to be created by a higher being to be assured that his cognitive faculties are reliable. So, much like primitive forms of the cosmological argument, this argument necessarily leads to an infinite regress of creators. Why not just stop with what we know exists?I'll now move on to address each of my opponent's arguments in turn.Proper FunctionalismMy opponent states proper functionalism as follows: "In order for a human to have knowledge, their beliefs must have been produced by cognitive faculties functioning properly according to a good design plan aimed at true-belief production." Skepticism is unavoidable on this account even if God exists, because even if God did design our cognitive faculties to arrive at truth, we do not know that God designed our cognitive faculties to arrive at truth. So proper functionalism leads to skepticism even if God exists, in the absence of a proof of the existence and benevolence of God, because in the absence of such a proof we do not know that we know anything. But the whole point of proper functionalism is to provide a reason to believe in God that does not depend on arguments or evidence.The validity of our cognitive faculties is a self evident axiom, and so it is the common property of naturalist and theist alike. It would be an odd person who was confident of the existence of God, but not of the validity of his faculties! We do not know how our faculties evolved to be reliable yet, and it may take 100 years to find out, but appealing to God is not a scientific answer.My opponent claims that naturalists cannot account for the normativity inherent in the concept of knowledge: "Plantinga's notions of "proper function" and "design plan" succeed where naturalistic accounts fail because Plantinga's notions provide a teleological normativity that naturalism cannot produce." However, there are naturalistic accounts of proper function. The notion of proper function in biology generally means something like serving the life of the organism. For example, the proper function of the heart is to pump blood because that is what serves the life of the organism it is a part of. So proper function with regard to cognition would mean doing what serves your life, which generally involves trying to get at the truth.He writes: "If naturalism were the case, then no human has cognitive faculties that function properly according to a good design plan aimed at true-belief production." No evidence is provided for this assertion, so I dismiss it as arbitrary. There is no reason to think that naturalism will not be able to explain why our cognitive faculties are so well adapted to get at the truth. The explanation will likely come from evolutionary biology, and certainly not from theology.The Argument from OmniscienceMy opponent's second argument is basically an argument from fallibility. If we assert that we know something without an omniscient being to back us up, then we can't be sure that it won't be undermined by facts or relationships between facts that we discover in the future. Knowledge therefore depends on the existence of an omniscient being.But as I've noted previously, this implies that we don't know that we know anything unless there is some independent reason to believe in God. If the existence of God is necessary for knowledge, and we have no reason to believe in God, then we have no way of knowing that we have knowledge. So my opponent has not avoided the necessity of providing independent arguments and evidence for God's existence with his epistemological reasoning.Moreover, the problem of fallibility is not solved by appealing to God. Even if God exists, we still have to rely on our own limited, finite faculties to learn about the world, and we will still make mistakes in our reasoning. (After all, my opponent presumably believes that atheists, Muslims, and the members of all other religions have made such a mistake despite God's omniscience.) Maybe my opponent believes that this is solved by appealing to God's revelation in the Bible, but the Bible is notoriously difficult to interpret in an uncontroversial fashion, which is the cause of all the different sects of Protestant Christianity. We are no more infallible about what the Bible says than we are about anything else.So how should we deal with fallibility?First of all, it simply isn't true that every conclusion, no matter how much evidence we have for it, is capable of being overturned someday. The law of identity is not going to be overturned by some new discovery. Moreover, the fundamental laws of science like Newton's laws and the theory of evolution are so well established that they are no longer regarded as provisional by mainstream scientists, although they may be modified with regard to some of their details. (Geocentrism was an idea from before the Scientific Revolution that was rapidly overturned when people decided to start looking at the world with their senses to find out what the world was like.)Secondly, we should regard "certain" as meaning "beyond a reasonable doubt," not as meaning that you can't imagine a claim being refuted someday. When you say that a claim is certain, what you mean - or ought to mean - is that all of the arguments and evidence support it and there are no significant arguments or evidence that undermine it. This is surely attainable in countless practical contexts. There's nothing wrong with going by the evidence that you have, and after all, that's what everyone is really doing in the end anyway.