User blog comment:B214/Talk About What You Want! 2/@comment-24098614-20140711102904/@comment-1359322-20140711104526

I believe that i explained this to you before, but i guess i'll do another exception. "“……[Truth Idea] is related to the “will” of the God from the Bible. The will of the deceased God sucks the ambition of the holder of this spear, and by responding to how strong the opponent is, it creates many effects and miracles…… That can be something with absolute power to destroy the opponent……or a blessing to the opponent to capture their hearts. –But the answer the [Truth Idea] gave to Sekiryuutei is silence. ……Then it means that Sekiryuutei won this match, and this spear wants to see Hyoudou Issei’s dream over mine…….”"

Explanation of the Truth Idea. It takes both Cao Cao's ambition of being a hero and Issei's dream of being a hero. So the result was it rejected Cao Cao's ambition because Cao Cao used the Medusa Eye, if Cao Cao had not rely on the Medusa eye and fought on as a "human", the Truth Idea won't have rejected him. As to why it chose Issei, it was because Issei's dream is to become a hero for the children of the Underworld and Issei's dream did not change, Issei is a Devil and accepts his role as a Devil while striving to become the hero of the children of the Underworld, so in the end the will chose Issei's dream.

One is a human who tried to be a hero for the humans and ended up going against his nature as a human, while another accepts his nature as a Devil and strives to become a hero for the Devil children. Cao Cao's ambition lost to Issei's dream the moment he went against his ideal of being a human who fights supernatural beings.