Comment by Vanitas on Raising Villains the Right Way - Chapter 31

Actually he is really an outer G** too. He just reincarnate as human and his divinity is the scary eye that you always see. He is like Furina lol Human body and his divinity. That's why people scared of him.

So essentially, it is heavily hinted that he reincarnated multiple times and he might even have reincarnated into this particular world in one of the previous timelines. He possibly reincarnated so many times that he completely lost contact with his divine self and is esentially an ordinary human. 
This timeline however, he reincarnated with memories of a previous life that gave him an advantage over things(It is heavily implied that he never actually played a game and that he actually lived through all the things in one of the previous timelines). He uses this knowledge to get ahead and prevent his own death and the destruction of the world.
There is a much wider plot at play here and the Outer Gods are just a single part of it. Him returing to the world and getting back in touch with his divine self has actually tipped the balance in favour of the Outer Gods and in theory the world itself. 
He will only get stronger and you will eventually see him climb up to somewhere between and 4th and 5th Circle and unleash spells that could esentially wipe out entire cities.

Yeah, I’ve thought about it. I mean, the “will” that the other og mentioned is most likely those eyes, so he must be one of them. Even before that, I had a feeling it was something otherworldly.

It is heavily hinted that he actually lived that life rather than experiencing it through a game. It is not confirmed though. He might have very well mixed up different things from one his past lives, since he had so many of them.