User blog comment:B214/Question and Answers/@comment-50.11.177.49-20130701230103/@comment-33447191-20130701232430

Humans are like the innocent bystanders throughout the progression of the story, while they aren't heavily focused on as a group, they still play a role in furthering the development of the Three Factions after their casualties from the Great War. Cao Cao was looking for specific individuals who possessed the "rare" Sacred Gears so that he could start a war with the Three Factions after learning of their truce and their subsequent alliance with one another.

As Azazel put it, "From the human point-of-view, Devils, Angels and Youkai are all enemies— They probably just view them as monsters. The alliance of the three Bible Factions seems to be the spark that lit them into action. Devils getting along with Fallen Angels is already shocking, but Heaven working with the Underworld is even more unbelievable. It isn't surprising that humans will feel threatened and angered at this unnatural turn of events." (Light Novel Volume 9, New Life)