It has been months since Sora arrived into this unknown world. During those months Sora learned why he was summoned. It seems like a powerful demon lord had taken over the demon lands and was getting ready for war with the human kingdoms.
In the beginning, Sora believed every word the Sliral Kingdom's people told him, but as he continued his journey heard the stories of different people, Sora figured out pieces of the truth.
There was indeed a powerful demon lord called Kretos, but he wasn't invading or planning a war with the human kingdoms, in fact, it seems like the opposite was happening. Based on the information he gathered by himself, the demon lord Kretos was not thinking of attacking the human kingdoms. In fact, it would seem like he was busy trying to quell the chaos in his own lands.