Let's have Earth-like planet with settlement of humanoids on tech level about 500 BCE. This culture is being visited by aliens and these aliens are being recognized as "Gods".
Aliens in question are not really decided, so for time being assume they are Grey aliens visiting in flying saucers. Reason of alien visit is simple: Check if that planet has unobtainium resources to mine.
So, these aliens do not really interact with such culture. All the aliens care about is that the people of planet do not disturb mining research.
After a year of researching, the aliens leave for good.
As noted above, the culture is left alone with feeling they have been visited by Gods. And that culture starts Cargo cult worshipping the Grey aliens.
My idea of this Cargo cult start is that the people observe aliens and are not able to grasp their technology. Therefore, the aliens are proclaimed as Gods
Now, such culture thrives to live and develops at same rate as ancient (eastern) Roman Empire would from year 500 BCE to year 500 CE on Earth. And my culture keeps that original Cargo cult and develops it.
The question is: What symbols will be preserved in such religion after 1000 years of worshipping?
You can assume same mental processes for these humanoids as if they were humans.