I would be interested in hearing how FFX ripped off this other game because that sounds incredibly unlikely.

I don't doubt there are similarities but is the plot of Grandia 2 that a guy who doesn't exist helps change a theocratic world trapped in an unending cycle of death by making the messianic summoner break away from her indoctrination and do what is actually right and not what she was taught?

I mean...if it is that, I'd honestly be very, very shocked.