The task of apologetics is the defense of Christianity. It is the communication of good reasons for the hope that lies within us (1 Pet. 3:15). All apologetic strategies can eventually be reduced to two methodologies: classical apologetics and presuppositional apologetics. Classical apologetics is described that way because it has held sway during much of the history of the church. It started with the Greek apologists in the patristic church who developed rational answers to the classical pagans' (and unbelieving Jews') objections to Christianity.
There are several prime variations of classical apologetics, but this method can be understood in a single brief sentence: It is possible to validate Christianity (at least with a high degree of probability) without presupposing Christianity. In philosophical terms, classical apologetics is foundationalist, rather than contextualist. Foundationalism is a view of knowledge positing that we begin from a certain basic, epistemologically unjustified axiom, and then gather knowledge on the basis of it. By contrast, contextualism holds that knowledge is a set of coherent beliefs. In order to gain knowledge, you need to "get inside" this "system." Classical apologists believe that you can start from a certain independent axiom independent, that is, of the Christian "system," and then work your way outward to at least a highly probable validation of Christianity.
For instance, a classical apologist may say, "You do not need to accept the God of the Bible and its Christian system in order to believe that the historical record of the Bible is largely true. There were many witnesses to Jesus' resurrection, and it is hard to believe that all of them, including some unbelievers and skeptics, were in cahoots to perpetrate a fraud. Therefore, it is highly probable that Jesus rose from the dead; and if He arose from the dead, He is most likely Who He claimed to be, the Son of God." This is one of the main historical arguments that classical apologists use.
Then there are experientialist arguments. The chief one is how Christianity has changed peoples' lives: "The great transformations of human characters throughout history; all of those martyrs willing to sacrifice their very lives for Jesus Christ; the joy, peace, and bliss of Christian experience, all these validate the Bible and Christianity."
Then, of course, there are empirical arguments. It is held that scientific evidence has great probative value. The two main variations of this argument are cosmological and archeological. The cosmological argument (and I am not referring to the specific "cosmological argument" for the existence of God) suggests that the very existence of the world and its evident design and nature verify the truth of the Bible and Christianity. Archeological arguments suggest that evidence like the physical remains from Noah's ark and from ancient cities mentioned in the Bible validate the truth of Christianity.
All variations of classical apologetics assert that one can be convinced of the truth of Christianity by some factor or factors independent of Christianity as a "system."
Presuppositional apologetics, by contrast, is distinctly contextualist. You can never validate the system until you "get into" the "system." There is no axiom independent of that system by which you can "argue up" to Christianity.
This in no way implies there is not evidence for Christianity. Presuppositional apologists do not, for example, argue that the accounts of witnesses to Jesus' resurrection are unimportant, or that attestation for Christianity's having changed individual lives is wanting, or that empirical data supporting the truth of the Bible and of Christianity is unimportant. Presuppositionalists simply argue that all of these much be understood within a Christian context. This is really just another way of saying that presuppositionalists believe that apologetics is fundamentally an ethical issue.
Those who do not accept Christianity, unbelievers, in other words, are not dissuaded from their unbelief by historical, experiential, or empirical evidence. They have their own context, an unbelieving context, within which to interpret this evidence. In the words of supreme presuppositional apologist, Cornelius Van Til, there are no "brute facts." "Facts" do not speak for themselves. Facts speak within a particular context. Within a skeptical, unbelieving context, for example, wide historical attestation of Christ's bodily resurrection can be easily explained without recourse to the infallibility of the Bible or the truth of Christianity. After all, the skeptics may claim, all sorts of strange things happen in the universe: "Perhaps Jesus Christ did rise from the dead. What does that prove?" Or, "We don't deny that people who accept Christianity enjoy changed lives. This is simply a new religious self-awareness. The same thing is true of many Buddhists, Muslims, and New Age practitioners." Or even, "Of course, we accept as readily as you classical apologists that there was a Noah's ark, which corresponds to what the book of Genesis teaches. But this doesn't mean that the Bible is the inspired Word of God' or that Christianity is true. It only proves that Genesis gives a reasonably accurate account of a large boat that finally came to rest in what is today Turkey."
Presuppositionalists know that men reject Christianity not for intellectual reasons, but for ethical reasons. Unbelievers will always be able to discount any evidence Christians may offer, since they can interpret that evidence within their own Christ-rejecting system.
This is to say that, ultimately, there are no true "foundationalists." All men are "contextualists," or presuppositionalists. Unbelievers interpret all of life in terms of their own covenant-breaking presuppositions. Christians interpret all of life in terms of their own covenant-keeping presuppositions. The variation is not in the "facts." The variation is in man's ethical nature.
The Presupposional Argument
What then is the actual argument of presuppositional apologetics? If it refuses to appeal to some independent source of validation, how can it argue at all? The answer to that question is as profound as it is simple. Presuppositional apologists do not contend that history and experience and archeology and physics and biology demonstrate that Christianity is "probably" true. Presuppositional apologists argue that unless one affirms Christianity, he can affirm nothing at all. Presuppositional apologetics is the ultimate reductio ad absurdum: it reduces all other perspectives to absurdity by demonstrating that by their very nature they are untenable. You have to presuppose the God of the Bible in order to know or presuppose anything at all.
Does this mean that unbelievers can know nothing? Of course not, but in order to know, they must borrow from the system of Christian truth. To put it another way, they can know nothing if they remain consistent with their own unbelieving presuppositions. They must, as Van Til says, act like the little girl who must sit on her Daddy's lap in order to slap him in the face. They must affirm God in order to deny Him.
Finally, presuppositional apologetics is the only valid apologetic method, because it refuses to compromise the sovereignty of God. Classical apologetics grants great autonomy to sinful man. It implies to the unbeliever, "You are quite free to stand as judge, prosecutor, and jury over God and over the Bible. If you hear all of the evidence and it meets your Christ-denying, covenant-breaking, rebellious standard, you are free to accept it. Of course, if it does not meet your standard, you are free to reject it." This is not merely silly; it is blasphemous. Men do not judge God and His Word; God and His Word judge men. Men do not know in order to believe; they believe in order to know. When we do God's will, then we learn of His doctrine (Jn. 7:17). When the Holy Spirit regenerates sinners, their eyes are open and they can then grasp the truth of the Bible and of Christianity. Until then, they are dead in trespasses and in sin, and their minds and understanding are darkened (Eph. 2:1-7).
Presuppositional apologetics alone upholds the standard of God's sovereignty in the preaching of the gospel.