Van Tillian Presuppositionalism is the Reformed and therefore Biblical apologetic school. This method proves that man cannot prove anything unless the Triune God of Scripture is presupposed; said otherwise, the Christian worldview must be presupposed (the Christian) or borrowed from (the non-Christian) in order to know anything. An Intelligible argument cannot be formulated from any assumption that denies the Triune God of Scripture. Cornelius Van Til in summarising his method stated that “The only proof for the existence of God is that without God you couldn't prove anything."
Van Tillian Presuppositionalism presupposes the existence of the Triune God of Scripture. There is no neutrality between the Christian and the Non-Christian, as the Christian accepts the truth of God, while the non-Christian suppress the truth of God in unrighteousness (Romans 1). God’s existence is ontologically necessary, and because man’s knowledge is dependent on God’s knowledge, our reasoning and logic must be in subjection to Scripture. The Christian interprets facts in light of the self-authenticating Scriptures; therefore, because Scripture has its foundation in the all-embracing plan of God, this God must be presupposed.
The Christian worldview is the only self-consistent worldview, as the atheist cannot account for the existence of anything without borrowing from the Christian worldview. Thus, the Triune God of Scripture is proven by “impossibility to the contrary”. Because the atheist denies the source of everything (God), they cannot account for the existence of anything; therefore, whenever the atheist uses logic, reasoning, morality etc., they are borrowing from the Christian worldview in doing so. Unless the existence of the Triune God of Scripture is presupposed, it is impossible to prove anything, as God is the necessary foundation for proof itself.
The Christian communicates to the Non-Christian the truths they supress; the non-Christian will either 1) continue supressing the truth of God in unrighteousness (Bahnsen compared this suppression to denying the existence of air while breathing it) or 2) by the Holy Spirit accept the truth of God that they formerly suppressed. In both outcomes, God remains sovereign: the non-Christian will continue to supress the truth of God, unless God appointed that specific time to effectually call them by His word and Spirit to grace and salvation by Jesus Christ, monergistically enlightening their minds spiritually and savingly to understand the things of God.
(C), J. Williams, July 2011.