If every denomination is skeptical of every other denomination, why shouldn't non-Christian outside observers be skeptical of all denominations?

To the best of my understanding, Christianity lacks a unified theory or epistemology. Instead, each denomination proposes its own framework (though calling these "theories" may be controversial; see [here](https://philosophy.stackexchange.com/q/118294/80226) and [here](https://philosophy.stackexchange.com/q/119181/80226)) that combines practical/experiential and non-practical/abstract elements. There are significant disagreements between denominations, each having its own epistemological basis—either explicitly defined or implicitly assumed—by which they often critique or reject the perspectives of others. To illustrate, here are some prominent examples: - **Jehovah's Witnesses** receive skepticism from other denominations because of their rejection of the Trinity, unique eschatological beliefs, and exclusive claim to doctrinal truth. - **Latter-day Saints (Mormons)** face skepticism for their additional scriptures like the Book of Mormon, beliefs in continuing revelation, and doctrines about God and the afterlife that differ from mainstream Christianity. - **Catholicism** includes beliefs such as Marian apparitions and Eucharistic miracles, which other denominations often view skeptically as unbiblical or exaggerated. - **Pentecostals and Charismatics** are critiqued for their emphasis on spiritual gifts like speaking in tongues, healing, and prophecy, which some see as lacking biblical or historical support. - **Calvinists** hold strong views on predestination and the lack of human free will in salvation, which others find incompatible with notions of divine justice and human responsibility. - **Eastern Orthodox** theology and practice differ from Western Christianity in areas like the Filioque controversy, the veneration of icons, and the concept of theosis, which others sometimes dismiss as overly mystical or traditionalist. Suppose an outside, non-Christian observer sympathetic to Lakatos' concept of [Scientific Research Programmes](https://en.wikipedia.org/wiki/Imre_Lakatos#Research_programmes) examines the landscape of deep Christian disagreements. Each Christian group is skeptical of every other group, with no shared research project advancing toward the truth—each simply holding its own beliefs regardless of what others believe. **What reason does Christianity offer this skeptical observer not to doubt all denominations simultaneously?** If there are *N* denominations, and each is already skeptical of the other *N-1* denominations, why would it be unreasonable for a skeptical observer to extend this skepticism to all *N*? After all, it’s merely adding one more denomination to the list. Alternatively, does Christianity present its own version of a progressive research programme (in line with Lakatos' definitions of *progressive* and *degenerative* programmes, as explained [here](https://en.wikipedia.org/wiki/Imre_Lakatos#Pseudoscience)) ? Could such a programme offer a pathway for an outside observer to eventually converge on certain truths that might align with one denomination or another? ----------------------------- Additional clarifications: * If an answer merely focuses on listing a minimal set of Christian tenets as the theoretical *hard core* of Christianity (in Lakatosian terms), that would still leave unanswered **why an outside skeptical observer would have any reason whatsoever to accept these hard core tenets in the first place** in light of the fact that (1) even within Christianity there are smaller groups that do not accept them, and (2) a simple listing of tenets doesn't explain how these tenets are useful to *make progress* in our understanding of reality. * Related to the previous point, it's important to keep in mind that a key concept that Lakatos retains from Popper is *falsifiability*, which means that a scientific research program has to make *falsifiable predictions* which are so in virtue of being testable empirically. Thus, **does Christianity share this scientific appreciation for the empirical testability of its claims to any extent whatsoever?**