Two Sources

Yet, despite its ease of mastery, the Gen Temple had managed to monopolize the technique for centuries. 

This was because they went to obsessive degrees to hide the novel training method for the technique. All memories of the training method were wiped from the minds of all those who went through it, ensuring that it could never be spread without the authorization of the Gen Temple.

Apparently, the training method for the technique was extremely novel because not a single force had managed to crack the key to mastering the Yin-Yang Shield. To the present day, it remained a secret that only the highest authorities of the Gen Temple were entrusted with.

This was despite the fact that many samples of the technique, such as Master Vertol, had spread throughout human civilization.

"You were… a part of the Gen Temple?" Rui cautiously asked.