What degree of certainty or justification can we have in biblical truth? And how does this compare to our certainty in scientific truth, or the truth of our own experiences?
In a colloquial sense, I have complete certainty in the truth of Christianity. But I would tend to think of certainty in more rigorous terms, where it is a function of probability of error. To be 100% certain, in these terms, is to say that I could not be wrong; that Christianity is necessarily true and that its contradiction is logically or metaphysically impossible (ie, issues in a logical contradiction, or violates some way that reality has to be).
I think it’s obvious that Christianity is neither logically nor metaphysically impossible. For example, God could have created the world of Narnia instead of our world. (That is much of the point of Narnia as fiction.) The distinctive truths of Christianity could be false, and I could be deceived about them.
However, the general truths of theistic belief are of the kind that they must be true. If God exists, he exists necessarily—there is no possible world without him. That being so, I believe I can have 100% certainty, in the more rigorous sense, that a being broadly like Yahweh exists. So this strongly underwrites the certainty I have in Christianity.
That said, my confidence in Christianity is not primarily based on philosophical reasoning; nor is it even based on God’s word, per se. It is based on the simple knowledge that Christianity is true—a belief both imparted and justified by the indwelling Spirit (cf 1 Corinthians 2; John 6:37-44). The philosophical reasoning, the prophecies, the historical facts—these things all attest to the truth I already know. Or, put another way perhaps, the Spirit confirms to me (not by words, but simply by direct experience of knowing) that the Bible is the word of God, that it is powerful to save, and so on. When I was converted, I began to experience Scripture differently. I began to know that it was true, and this was not something produced from within me. The various other proofs of Christianity simply confirmed and cemented this knowledge—but they are critically important in this role, because they demonstrate that I am not merely self-deceived as, say, Mormons are with their experience of the burning in the bosom.
In terms of more general experience, I have as much confidence in it as I have in my experience that Christianity is true. I recognize that not every detail is infallible, but that the broad strokes are of such a nature that I cannot doubt them without doubting my own sanity. We should expect this, since God created us to live in, interact with, and rule over our world—which requires that we have an accurate perception of it.
However, what we experience, and how we interpret that experience, are not the same thing. Interpretation is often the issue. This gets not only to the issue of some kinds of theology, but also especially to the question of our confidence in science. I trust science a great deal, since I know that God created the world to be orderly and uniform and understandable—and that is what science is all about. (Indeed, atheistic science is incoherent because of this.) But having confidence in science as a method is not the same as having confidence in the application of that method to any particular case. Indeed, the history of science is the history of being wrong—that is how science works. The entire idea of science is about falsifying a hypothesis. And the more assumptions and the fewer direct observations are involved, the more tenuous the conclusions tend to be. So I have a great deal of confidence in, say, quantum theories that we rely on every day to publish blogs. I have much less confidence in speculative theories like evolutionary psychology, when there are equally plausible alternative explanations for the data which are merely excluded on the basis of philosophical assumptions (ie, naturalism).
This also gets to a serious question of the right approach to interpreting experience more generally again. Supernatural events are experienced frequently, by people all over the world (for example, I find particularly interesting sleep paralysis with a malevolent presence). Mainstream scientific explanations of these experiences simply fail, shoehorning them into neurological categories like dreams or pathological hallucinations, and bolstering these explanations with circular theories like the cultural source hypothesis. These are easily falsified. However, at the same time, a lot of Christians find these kinds of experiences troubling because they are not mentioned in the Bible (or they think they aren’t; cf Job 4:13-16). So they tend to take a secular, disbelieving approach to them. But our worldview needs to be flexible enough to take experience seriously even when it isn’t clearly described or predicted in Scripture—which means our understanding of the place of Scripture itself needs to be tensile enough to cope with this. We should be able to adjust our theology based on experience, even when that experience is unexpected or troubling—but we need to have some core principles in place at the same time to keep us bounded, or we will fall into error (I’m thinking especially of Pentecostalism here).
In other words, we need to be able to carefully and thoughtfully systematize and integrate real-world experience and scriptural data to form a coherent, consistent worldview that accommodates both without falling apart. That isn’t easy, necessarily, but I believe it is possible because I have confidence that Scripture is entirely true—and since truth is consistent, our true interpretations of veridical experiences will line up entirely with our true interpretations of Scripture.