Yuusha no Furi mo Raku ja Nai. Riyuu? Ore ga Kami dakara
One of the countless Gods, Keika (Hiko-no-Mikoto) failed at gaining believers in Japan. He got thrown off to another world when he was going back to Heaven in disappointment.
That other world, Alexsild has been mostly seized by the demon king. Mysteriously, Keika who should have been invisible to other people could be seen normally like a normal human. And then, a princess knight told him that he could ascend to Godhood if he piled up achievements as a hero.
Keika decided to defeat the demon king and become a God in that world.