Religion is a philosophy--a system of concepts (mental representations, ideas of people and things--objects) and principles (mental representations/ideas of events--relationships between/among people and/or things), and techniques (applications of the concepts and principles)--which includes a belief in the existence of gods, supernatural beings, mystical beings.
If religion is a philosophy which includes a belief in the existence of gods, then the fundamental problem of religion is proving that gods exist.
What is a god?
Among the characteristics of beings which qualify as gods are (A) more knowledge than humans and (B) more capabilites for using knowledge than humans.
The second fundamental problem of religion is specifying what shall be proof of the existence of gods.
Proof consists of (A) Physical Evidence (people/things/events comprised of matter/energy who/which are observable by the perceptual senses of sight/hearing/touch/smell/taste directly or indirectly by their directly observable effects upon directly observable people/things/events; (B) Credible Eyewitness Reports of physical evidence from individuals who are reliable and therefore credible because they do not have a personal history of lying or otherwise of being unreliable/not credible and no one is aware of any motive they may have for lying, and these reports must be corroborated by reliable/credible corroborating reports from reliable/credible corroborators; and/or (C) Valid Logical Arguments consisting of premises which are verifiable/falsifiable/verified by physical evidence and therefore true and which lead to relevant conclusions which are valid if the premises are valid and which are true if the premises are true.
Note that B/Eyewitness Reports and C/Logical Arguments both rely upon physical evidence, which is A/Physical Evidence, thus A/Physical Evidence is the one-and-only acceptable proof of the existence of gods.
Eyewitness reports of physical evidence of gods have not been credible because the people making such reports have not been reliable/credible and/or the reports have not been corroborated by reliable/credible corroborators, and the reports have been determined to have contradictions and factual errors that do not support any claim of internal reliability/credibility.
Loigical arguments for the existence of gods have all been shown to have premises which are not verifiable/falsifiable/verified by physical evidence and conclusions which are false because the premises have not been verified to be true.
What is physical evidence of the existence of gods?
The only acceptable physical evidence which can serve as proof of the existence of gods are the gods themselves. They must appear in a form humans can perceive directly with the normal human perceptual senses of sight/hearing/touch/smell/taste, and they must perform stunts which can be observed directly and which prove they have more knowledge and capabilities for using that knowledge than humans.
Physical evidence can sometimes be perceived indirectly by directly observable effects upon directly observable people/things/events, but the problem herein is proving that the directly observable effects upon directly observable people/things/events are truly caused by the people/things/events claimed to be the indirectly observable causes.
Thus far, in contemporary times, no one has captured a god, tortured it until it confessed it is a god, and forced it to perform stunts which prove it has more knowledge and capabilities than humans, therefore, because of the absence of physical evidence of the gods--the gods themselves, no human being has any rational justification for belief in the existence of gods.
Nevertheless, the absence of physical evidence of the existence of gods is not proof that the gods do not exist.
Anyone can claim that the gods are unobservable unobservables, and no one can prove by physical evidence that he is wrong.
Because gods are claimed to be unobservable unobservables, anyone can claim he has knowledge of the existence of a god and of what that god wants from humans and no one can prove beyond a doubt by physical evidence that he is lying, that he has not seen a god, and that he has no knowledge of what the god wants from humans, hence, concerning the claims of religionists of the existence of gods and of what the gods want from humans, anything goes.
He who asserts must prove.
The person making a claim must prove the claim by physical evidence, eyewitness reports, or/and logical arguments.
The technique for recognizing that an individual is lying when he claims he has knowledge of the existence of a god and of what that god wants from human beings is to demand directly observable physical evidence of that god—the god itself, and to reject any claims of eyewitness reports that are not completely reliable/credible (because the witnesses and the corroborators are reliable/credible and the reports contain no contradictions or factual errors which do not support a claim of internal reliability/credibility) and/or logical arguments unless the premises are supported by directly observable physical evidence and lead logically to relevant conclusions which are valid if they are relevant to the premises/the premises are relevant to the conclusions and which are true if the premises are true/verified by physical evidence.
If (P1) there is no physical evidence consisting of gods performing stunts that prove they are gods because they have more knowledge/capabilities than humans, (P2) no reliable/credible eyewitness reports of physical evidence of the existence of gods, and/or (P3) no valid logical arguments for the existence of gods whose conclusions are true because they are relevant to their conclusions and their premises are relevant to their conclusions and are verifiable, falsifiable, and verified by physical evidence to be true, then (Q1) no person has a valid reason to conclude and thereby claim that gods exist and (Q2) no person has any reason to claim that gods must be included in personal, organizational and political (public) decision-making.