Resonance (霊応力 reinouryoku?, "Spirit Stress") is a special term in Tales of Zestiria and Tales of Berseria. It gives one the innate ability to perceive seraphim/malakhim, and malevolence.
Description
Resonance is the result of a human having high spiritual power. It causes a person to be able to perceive malakhim/seraphim, although seeing one requires more innate spiritual power than just hearing one. The spiritual power also plays part in perceiving daemons/hellions as these require lower resonance than the celestial beings. This results in many legends about monsters and demons, there are few legends about the seraphim.
How a human can have high spiritual power is uncertain, however it appears it is not something that one can be just born with it, but rather spending the forming years in the presence of a seraph, preferably in an untainted environment. In Tales of Zestiria, Sorey was raised in a seraphim village which helped him build strong resonance, while Rose attains it due to years of abuse from Dezel, but her realistic personality has caused her resonance to be inactive. "Mere" great faith in seraphim also does not seem help in achieving high spiritual power as seen in case of Alisha Diphda.
Interestingly though, despite being merely a fetus, the unborn child of Celica Crowe and Artorius Collbrande had innate resonance before being devoured by Innominat and reincarnated into Laphicet.
History
Tales of Zestiria
As of the events of Tales of Zestiria, seraphim has faded into legend, although belief in them is kept alive by the church. Despite that, they are generally seen as delusions.
Tales of Berseria
During two Scarlet Nights humanity's resonance was bolstered: the first was the Opening which allowed humans to see daemons, while the other was the Advent that result in perception of malakhim by human eyes.