How about a divine revelation that specified that the distance around a circular brass bowl ten cubits across would be thirty-one and forty-one one-hundredths cubits?
And further stating that this is a holy number that may be found hidden across the heavens and the Earth by those with the wisdom to see?
Yeah, I know that fails because it would only be of importance to people who know the value and significance of pi, which makes it non-obvious to earlier folks – though giving a measurement that precise when all the other people around were at best using ten cubits to thirty-one cubits ought to make people sit up and go, “Maybe this info source really is God”.
The thing is, then, that for something to make God’s reality obvious it would have to do so for all humans however far back God would want to start, and to hunter-gatherers the ratio of a circle’s circumference to its diameter isn’t even worthy of notice; it doesn’t become relevant until there’s a culture that can afford to use valuable resources on making things as large as ten cubits across and can measure precisely enough to get forty-one on-hundredths of a cubit determined.
My thoughts go to astronomy, but there’s another problem there: people in the northern part of the world won’t be seeing the same stars as people in the south, so anything in the skies doesn’t qualify as obvious unless it’s duplicated for both hemispheres. So something identical around each pole star, but then what sort of star pattern would make God’s existence obvious? Everything I can think of requires a society to have reached the level of pictograms at the very least, so pre-writing societies are excluded.
With one exception: planets. It’s not possible to have a planet in a given solar system that is never visible from either hemisphere on a different planet. The question then is: what attribute could be given to a planet that would make God’s reality obvious? It has to be round, to fit the planet; it has to be symmetrical so it has the same appearance to both hemispheres, it has to be obvious…
That last leads to the requirement that this planet has to be large enough to not merely show a disk to the bare eye but to show enough of a disk that some broad detail is discernable. That means something much larger than Jupiter since it will need to take up a chunk of the night sky something like a third of the diameter of the moon. I’ve lost too many math skills to run the numbers on that off the top of my head, so I’ll leave that aside and move to what attribute the planet will need–
The thing that comes to mind is a human eye: if a planet showing a disk to the bare human eye large enough to show anything, simple is best, and the only thing I can think of that would have obvious impact to all human cultures ever is to put an “eye in the sky” orbiting out there. Of course the planet will be rotating on its axis (I can’t conceive of a gas giant that’s tidally locked and has a stable surface pattern), so the eye will come into view and depart from view on a regular schedule, but that’s no trouble; an eye in the heavens is still an eye in the heavens (though if it comes and goes its not quite as indicative of a Creator deity since one would be expected to watch all the time… maybe two planets that appear identical in size and have identical eyes?).
At any rate, I’d think that a human eye looking down from the heavens would announce to every primitive society that Someone Is Watching. That should work long enough that by the time writing comes along God could specify that four-digit precision for pi.