The key discriminator of this school is that it maintains that the Christian apologist must assume the truth of the supernatural revelation contained in the Bible (that is, the Christian worldview) because there can be no set of neutral assumptions from which to reason with a non-Christian. In other words, presuppositionalists say that a Christian cannot consistently declare his belief in the necessary existence of the God of the Bible and simultaneously argue on the basis of a different set of assumptions (presumably those of the non-Christian) in which God may or may not exist.
Comparison with other schools of apologetics
Presuppositionalists contrast their approach with the other schools of Christian apologetics by describing them as assuming that the world is intelligible apart from belief in the existence of God and then arguing on purportedly neutral grounds to support trusting the Christian Scriptures and the existence of God. Thomas Aquinas himself insists that many crucial truths can only be known through scripture, and none of his arguments are intended to show the entire Christian picture. Specifically, presuppositionalists describe Thomistic (also "Traditional" or "Classical") apologetics as concentrating on the first aspect of apologetics with its logical proofs for the existence of God, simply assuming common ground with the non-Christian and utilizing a piece-by-piece methodology. In this scheme, the common foundation of neutral brute facts leads to a generic concept of deity, then to the various characteristics of the Christian God as revealed in Scripture, and so forth. Piece-by-piece, Christian theology is built up from a neutral common ground.