I read this many years ago, so I can't remember much about it. One of the antagonists of the series was God and it turns later in the story that God wasn't actually immortal, but that a new host is chosen to be the new God after a certain number of years. The mc's love interest(?)/one of the female characters is chosen as the next God by the current God. It turns out she knew about this all along when we thought she was just a regular person.

I'm literally a rock.

Sounds like Outlanders or Mahou Tsukai Kurohime.

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