Jimi na Kensei wa Sore Demo Saikyou desu

Shirokuro Sansui, a Japanese student, was accidentally killed by God, then sent to another world in apology. There, he received training from a sage for 500 years in order to become "the strongest." After training enough to be considered the strongest by the world's current standards, he happened upon a baby near his training grounds, thus beginning his otherworld adventure. His training was normal, his moves are boring... even though he's the strongest person in the world, everything else about him is just plain. The curtain opens on the sword saint who fights with just a normal wooden sword.

Official English LN release: J-Novel Club