Doesn't quite answer the spirit of the question: why would any person when present the Nicene Creed, versus the Shamanic one I proposed see that one was true and one was not? I don't expect that anyone would accept it without previous engagement with the Church, with the Scriptures, etc. The Creed is a summary. It is not the whole.
If it is only through ones exposure, it being repeated numerous times and the culture that surrounds it, then how can you know the Islander Shaman is wrong and your Priest is right? How can you know you are not only believing in a false doctrine because of your birth circumstance?
Because I was brought up in the West in the modern era with all the advantages of a fine education, exposure to many cultures, freedom of speech, freedom of and from religion, so that I can ask any questions i wish to ask, of anyone I wish to ask them of. Not everyone has those advantages.