In both politics and religion we often hear that something is undeniable. In politics the undeniable "facts" are justified as simply a matter of "commonsense". By implication, the validity of such political "truth" would only be denied by a fool or a scoundrel. Religiously undeniable "truth", however, is rarely sold as "commonsense. Instead, we generally hear that such "truth" is divinely revealed and therefore cannot be challenged because of its infallible source.
Any serious student of human history knows that undeniable truth, especially of the "commonsense" variety, is a very shaky notion. Repeatedly, a "commonsense", undeniable fact has melted away, when new "incontrovertible" evidence overwhelmed what was previously believed to be self evident.
A perfect and often cited example of this is the account of the experiment with gravity conducted by Galileo at the Tower of Pisa. It is quite easy to imagine how, those before Galileo, assumed that the speed of a falling object depended on its weight. After all the harder one pushed on an object the faster it moved. Weight and force seemed equivalent. Heavier objects were subject to a great force and therefore "logically" moved faster in freefall. What could be more "obvious", everyone thought. But, lo and behold, Galileo demonstrated that "commonsense" was totally wrong. Similar demonstrations have been repeated many times, more recently by Einstein and others in the field of physics.
Religious infallible truth faces the added hurdle of dealing with issues outside the realm of physical experimentation and which by their very nature often do not appear supportable by so called commonsense evaluation. The "undeniable facts" of religion are therefore inherently a matter of faith. In the case of our prevailing religion, institutional Christianity, this is faith in the divine origin of the Bible as we have it and faith in our ability to understand and interpret that revelation.
Since faith rather than rational evaluation underlies religious truth, that faith must have a basis. Faith based religious truth may be comfortable because it is widely held. It may have been adopted in the impressionable stage of early childhood and therefore became deeply embedded in our subconscious mind. It may be only loosely held as true because one's life is not dependent on such truth, and it remains unchallenged because it is irrelevant in the current context. All these and other reasons allow "undeniable religious truth" to exist and even flourish though its basis is anything but undeniable.
Orthodox Christianity's claim to be the only people acceptable to God, however, implies that its theology is based on undeniable truth. A theology, which insists that all must adhere to it because God demand as much, must substantiate that claim in some sort of readily acceptable, i.e. provable way. How else could all people ever be convinced of its validity? Ironically though, we know that the tenets of Christian Orthodoxy have been argued for centuries and still are. Undeniable truth which is continuously debated is hardly undeniable.
I believe we have every reason to remain skeptical of even the most seemingly solid truths, political and religious. Too much of what is said to be undeniable turns out to be a house of cards under serious scrutiny.