Hypothetical scenario.
Premise 1: Everyone on the planet that believes in a god believed in the same god and practiced the same religion.
Premise 2: The religion promises a savior for the planet that will one day come and solve all the negative social dynamics of the planet once a critical mass of people did believe.
Premise 3: There is no physical evidence that said god ever existed except a prophesy carved in a cave that has been preserved by those who do believe for centuries.
Would that hypothetical world make the concept of "god" (as opposed to the religion) more credible to anyone?
Psychologists, behavioural scientists and sociologists might expect to find stronger belief, more believers and agnostics, and less atheists than would otherwise be the case because of greater societal pressure than in a more diverse cultural setting where the population is highly educated.
I don't see how that makes it more credible. It may, big emphasis on may, make that planet have less religious warfare. At some point, just like Christianity and Islam, splinter groups will form.