There's a debate within those who believe in some sort of rational defense of Christianity about how it should be done. The main lines of the debate are between what I call the classical apologetics view and the presuppositional view. I've never understood the presuppositionalist position, and all the arguments I've ever seen in favor of it seem so bad to me that I have to think there's something to the view beyond what people seem to me to be saying, but I've still seen no evidence that anyone has a better statement of the view and its claims than the bad ones I've so far seen. I've finally gotten around to putting together my thoughts on why I think presuppositionalism is fundamentally mistaken.
Presuppositionalists claim that classical apologetics has four flaws. First, it diminishes God's glory by conceding that God might not exist. Second, it doesn't prove enough by not proving every claim Christianity makes. Third, traditional apologetics denies God's sovereignty in salvation by thinking arguments can save people. Fourth, evangelism must be done in community, with the whole gospel in mind, with all of biblical revelation as a backdrop. Using arguments from classical apologetics in a vacuum is a failure to engage people with the gospel.
It's easy to see without too much thought why these are all silly criticisms. On conceding that God might not exist, I'm not sure why presuppositionalists think this. If I offer a cosmological argument for God's existence based on premises I think a naturalist will believe, am I conceding that God might not exist? I believe fully that God exists. My offering this argument to someone who doesn't concedes nothing. In fact, it seeks to prove that God does exist or at least that a being unlike any a naturalist believes in exists, a being who does fit the characteristics of the God Christianity says revealed himself through the Bible and through Jesus of Nazareth. How can offering an argument for the existence of God count as conceding that God doesn't exist?
The best I've seen at answering this question is that starting from premises that don't assume God's existence somehow counts as conceding that God doesn't exist. I'm not sure why. It just means the argument isn't circular. It doesn't mean the person offering the argument ceases to believe in God. It means the person offering the argument thinks premises held by someone who doesn't believe in God will commit them to the existence of God. Hey! Isn't that the main method of the presuppositionalist? The presuppositionalist strategy, insofar as any arguments at all are offered, is to offer transcendental arguments for God's existence. This means establishing that something we all believe in (e.g. reason, mathematics, the existence of the universe, morality, etc.) already assumes God. The problem with presuppositionalists' doing this is that it turns out to be a deductively valid argument in disguise once you fill in the missing premise (i.e. presupposition):
1. Belief X assumes God exists.
2. Belief X is true.
3. Therefore, God exists.
This is the same form of argument that philosophers have dubbed the cosmological argument:
1. Belief in contingent beings assumes God exists.
2. There are contingent beings. (So it's true that belief in them is true.)
3. Therefore, God exists.
Then you just have to argue for premise 1 by saying that an infinite series of contingent beings, each explained by a prior one, still has no explanation for why any contingent beings exist at all. So we must have a being that's self-existent to explain why any contingent beings exist. God is the best candidate for such a being. Presuppositionalists think all that extra stuff won't be granted by the naturalist. Well, some of it won't, but neither will all the steps given by the presuppositionalist to support premise 1 of the first argument for the claim that morality, logic, or knowledge requires God's existence. How are the arguments any different? Yet the presuppositionalist continues to criticize the classical apologist for doing the very thing the presuppositionalist does and considers a good argument.
The second problem is one noted by almost every classical apologist. Even if I establish that there's a self-existent being through the cosmological argument, a being greater than any other conceivable being via the ontological argument, a creator and designer of the universe through the teleolological argument, and a good being responsible for grounding morality in its own being through the moral argument, I have still not established all the divine attributes revealed about God in scripture. If I add to that historical evidences for the resurrection of Jesus, the reliability of the Bible, and the success of the church as a witness to the work of the Holy Spirit in history, it still falls short of proving every truth Christianity offers. So? If I could show all that, wouldn't it be enough to convince most philosophers who want evidence for God? The fact is that these arguments aren't intended to prove the existence of God and everything Christians believe about God. What they're supposed to do is show that there can be some rational support for some of the claims of Christianity, particularly ones questioned by naturalistic philosophers, historians, biblical scholars, etc. Pursuit of truth is a good thing, and why not see what we can discover through the ability of reason given to us by God, fallible and finite as it is?
The presuppositionalist will still insist that many arguments given by the classical approach involve a conclusion that God's existence is only likely and thus possibly false. It's true that if the conclusion is that God's existence is merely likely, then that will imply that it's possible God's doesn't exist. However, no argument I've seen shows that God's existence is merely likely. There are arguments intended to show that God might existence. There aren't any to show that it's merely probable that God exists. To show that you'd need to show two things, first that it's probable that God exists, and second that it's no more than probable that God exists. I'm not sure how the classical arguments are supposed to show the second thing. They're just supposed to show the first thing. Evidence that something is likely does not count as evidence that the thing might not exist. Something's being likely is consistent with it being absolutely necessary, which might be shown by another argument. The real confusion here, I think, is between reality and our knowledge of it, a common mistake by people discussing philosophical issues who aren't trained to spot such errors.
The third problem presented for presuppositionalism is a problem for any apologist. People aren't going to be convinced even by sound arguments if there's a way for them to deny a premise. Sometimes this point is confused with the previous one. That arguments don't always lead to every truth about God and that some people resist what the arguments do say are different points. My main resistance to this argument is that no classical apologist thinks arguments save people. That's a straw man view of the classical position. Arguments play a role in some people's coming to faith in the same way that people's looking out on creation and seeing the wonders of the universe play a role. It's the same way hearing testimonies of what God has done can play a role. It's a role that's insufficient without the work of the Holy Spirit in someone's heart. It's a role that's not necessary to experience before being transformed by God's grace, but sometimes it is a step in that process. Someone who has serious intellectual objections to believing in God who then sees classical apologists' responses to the problem of evil and the no-evidence argument may be more intellectually open to Christianity. This may be one step in God's bringing this person to understanding and appreciating the gospel message.
The fourth argument also misunderstands what classical apologetics is about. The classical apologist sees herself as playing part of a role that should be in the context of the greater evangelistic mission of the church. Ideally the apologist is also engaged in presenting and explaining the gospel in addition to living out the Christian life in the view of those who will be hearing the apologetical arguments. In fact, that whole context is part of classical apologetics. The arguments are just a piece of it. So again we've found presuppositionalism using a straw man position as a whipping boy.
So those are the four main presuppositionalist arguments, and they all seem bad. I have one additional problem with presuppositionalism based on the strategies presuppositionalists can allow. The main presuppositionalist strategy, as far as I can discern, is to insist that we start with God's existence and the truth of Christianity. We have to assume that from the start and somehow show the unbeliever the falsity of believing otherwise. How do we do that? One strategy is the transcendental argument I already gave, which is basically a classical approach in disguise, so that won't do. The only thing left is to do what they often give as the very basis of presuppositionalism. The proper kind of argument, according to presuppositionalism, is not to set aside God's existence and Christianity for the sake of argument. Instead, bring all of it to the table. In short, we need to present an argument of this form:
1. Christianity is true -- all of it.
2. Therefore Christianity is true -- all of it.
That's about as viciously circular as an argument can get. It's so ridiculous that it's not even worth spending time on. Yet this is what presuppositionalists seem to think can count as a good argument for Christianity. I have myself given an argument for Christianity based on the larger perspective of what Christianity says about itself, one I don't think is viciously circular, but I know that's not what these people have in mind. They want to say my argument is as bad as any classical argument.
This is why I still haven't been able to figure out what presuppositionalists think their strategy amounts to. As far as I know, it differs little from the postmodernist who thinks we can present our perspective to get it recognized by those who have a different perspective and then sit around talking about how we all have different presuppositions and reveling in the fact that we haven't gotten anywhere in our discussion. I just don't see how any of this is supposed to count as apologetics. So presuppositionalism is one of two things -- classical apologetics in disguise or nothing at all. Different people who hold to the title may be different ones, and one person may occupy both spots at different times. Either way, there's no real view here to begin with.