Jimi na Kensei wa Soredemo Saikyou desu / Простой на вид святой меч, но в то же время самый сильный

A Japanese student named Shirokuro Sansui died because of God's mistake and was teleported to a different world. He claimed that he wishes to become 'the strongest,' so he received a referral letter addressed to a sage from God and 500 years of training from the sage. When he reached a stage where he could be called 'the strongest' in the common people's standard, he stumbles upon a baby near his training grounds, and there, he finally starts his adventure in another world. His training was simple, his techniques are simple, he's the strongest but he looks simple. With an ordinary wooden sword in hand, the fight of the simple-looking Sword Saint has just begun.

(Source: Novel Updates)