What are the reasons you don't believe this theory?

[Zenith]

Did you see him coming from a portal? Could we actually detect and measure this portal?

well, the theory would be that the portal has been opened when they decided to go (after the universe has been set in motion, perhaps just after the evolution process has began) and closed just after they passed through it. When this particular god has come, a portal has been opened far in the universe, so it could have not been detected and closed just after this god entered our world. Some time after this event, he has appeared in front of me, and told me these things.

Do you have subjective evidence (since that would be the most you have) that it is both productive and harmonious with the information presented that what you were told was true? Is the only potential for productive usefulness of believing postmortem?

that would require me to imagine a lot of things to be productive, etc. It's pretty hard to understand what you mean by "evidence ... that it is ... harmonious with the information presented that what you were told was true?" - as a matter of fact, I cannot understand this sentence. The same with this question: "Is the only potential for productive usefulness of believing postmortem?".

The premise is that the theory is logical, accords with modern science. Also, as every religion has benefits of believing it, many or all of which cannot be tested (the gods will protect you if you worship them faithfully) or cannot be tested before converting, and a very desirable paradise is prepared for the people that faithfully serve the gods.

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what are the reasons you don't?

Same reason I don't believe in any kind of god. No evidence, no credibility and is an extremely high improbability. Perfectly good reasons to reject it.

By the way, just curios... what evidence would you expect in order to believe this theory? Would you need to see the god too? Would you need to see some supernatural phenomena in order to believe (e.g. to see me doing something naturally impossible, special effects, perhaps foretell future, etc.)? Or what exactly is that evidence you expect?