The defining use of an `a priori is based upon the reasoning of self-evidence without needing any prior experience. An `a posteriori is an assertion based on prior experience. The application of Christian faith often vacillates between these two thought processes and thus, causing confusion in what or how one defines “faith.” Therefore, an understanding of the two adjectives (adverbial usage also) is in order.
The simplest explanation for an `a priori is found in natural law that governs the universe. Intrinsic to this is mathematics. In spite of contemporary attempts to redefine the terms, 1+1 will always equal 2, as 5+5 will equal 10 and 10+10 will equal 20. This is determined before we deliberate the evidences. In fact, it will be true regardless of whether or not one is aware of its existence. However, an `a posteriori is rooted in mankind’s observations and experiences. Therefore, while an `a priori may be affirmed by an `a posteriori, it does not require the `a posteriori to make it true.
Christian “faith” has largely been presented as something that requires a belief or believer(s) in order to exist. Christian apologetics often die at its own sword by arrogantly presenting half-cocked arguments in the face of well thought out paradigms from millennia old religions and hammered-out philosophies. Sufficient evidences for Christianity are often presented more on the inclination of opinion, rather than absolutes. Moreover, ethereal abstracts seem more attractive in an effort that appears to have a greater concern for winning the day through confounding, as opposed to convincing another of a universal truth.
For example, the United States Declaration of Independence states that the “Laws of Nature” and “Nature’s God” are the basis for what entitled them to make a separation from the “political bands” to another. Therefore, it is not merely a wish of a people. It is an absolute that is so essentially true, the next paragraph begins with, “We hold these truths to be self-evident.” One may argue that the statement still is a matter of a belief to one group of people, because another can be produced that disagrees with the premise and conclusion. However, the evidence of any person’s demand for freedom remains a universal truth; this is how these rights can be declared as “unalienable,” meaning, they cannot be removed. This is a classic `a priori statement.
Presenting the gospel to an unbeliever on the foundation of what one “believes,” even when given in light of personal experiences (`a posteriori), leaves the subjective door wide open and has a tendency to disempower its truth. While the `a posteriori can be an excellent form of testimony and witness to the `a priori, the universal truth must be presented and held as the absolute truth. Christians have too easily yielded massive grounds concerning the Gospel in the name of philosophy, psychology, science, sociology, and tolerance thereof, all for the sake of acceptance and approval, in fear of unmerited labels.
Additionally, Christians can be tempted to overextend themselves into a form of hyper-spiritualism when sharing the Gospel solely upon an `a posteriori basis. In efforts to convince the unbeliever of the testimony, approaches to sensationalism are attempted which typically end up in alienation. If this sounds far-fetched, consider how often the gospel is actually shared from the pattern of created order (`a priori) as opposed to a personal conversion experience (`a posteriori). When the Apostle Paul engaged with educated unbelievers on Mars Hill in Acts 17, he began with the `a priori of creation and built his argument from there.
However, a word of caution is advisable. When presenting an `a priori, the truth of the statement must not be padded with preference or even experience. It must be left to stand on its own with no fences, walls, or minefields. Any such doing reflects fear that it cannot withstand question or scrutiny. Creation, order, the purpose of mankind’s existence and the need for salvation in a broken world are truths that are indeed, self-evident, as is the God who holds the answer to all of them.