Aiwass and Oswald returned very late.
Bishop Mathers' time teaching the Sacred Arts was not very long.
Once the necessary information had been imparted, what remained was the persistent practice.
After all, such things cannot be rushed. In the end, these mystical powers are only resonated with because Transcendents walk their own Paths, and because they align with the philosophical essence of their Paths.
The reason why studying, practicing, and using Mystical Skills can raise one's Extraordinary Profession level is that while Transcendents perform these skills, they are essentially practicing their own Paths.
Since it's about practice, it surely cannot be achieved overnight.
Unless someone like Aiwass has made a breakthrough.
Apart from "Basic Prayer," Aiwass had quietly mastered the remaining three types of Sacred Arts. Prayer didn't need to be taught specifically since it's part of Theological Seminary's curriculum.