Ji Changsheng initially did not understand the major sects of this world.
However, when he was framed by Bai Guanlin earlier... scratch that, when he exposed Bai Guanlin's sinister plot, he had heard of Danqing Pavilion from the temple master.
The note Bai Guanlin used to frame Ji Changsheng was written with Transforming Ink, a special product of Danqing Pavilion which was very precious. Even many elders of Mystic Temple did not have reserves.
At that time, Ji Changsheng knew that Danqing Pavilion was no ordinary power.
So he specifically asked Ying Hongye and Li Daoyan, and confirmed that Danqing Pavilion was indeed one of the righteous immortal sects that are on par with the Mystic Temple.
Their power is not to be underestimated - as always, if you disregard Fairy Linglong and the Mysterious Yellow Virtue Linglong Pagoda, the force of Danqing Pavilion might not be weaker than Mystic Temple.