On May 27th, (1926) Meher Baba discoursed on the Personal and Impersonal sides of God:

The foremost and highest object in life is personally serving a Sadguru and complete surrenderance to him. The second is constant meditation on the divine name. The third is selfless service, and the fourth is the offering of worship (prayer). Serving a Sadguru is like taking a course for a Master’s degree; meditating on the divine name is like studying for a Bachelor’s degree; performing selfless service is like obtaining a high school diploma, and offering prayers is like learning the alphabet. Once one attains the Bachelor’s degree, it does not take much time to attain the Master’s degree.

Were someone to ask me who is greater Personal or Impersonal God, Tukaram or Bhagwan, Zarathustra or Ahuramazda, Jesus or God the Father, I would definitely answer that Tukaram, Zarathustra and Jesus are greater. In fact, they are the Greatest of the Great because by being a Sadguru or the Avatar they render infinite service to the universe and suffer infinitely by taking upon themselves the burden of the world’s infinite amount of sanskaras. Undoubtedly, a conscious divine person such as Tukaram or Zarathustra, compared to the unconscious Bhagwan or Ahuramazda (formless God), is definitely greater!

Zarathustra was actually God in human form – an Avatar. In order to work in creation, he had to come down as man among illiterate, fanatical and hot-tempered humanity. Had Zarathustra told them to worship him, they would have denounced, harassed and murdered him. They would have thought him an enormous egotist and absolutely crazy. So he taught them to pray to formless God. But in reality, by worshiping formless God, they were worshiping him; and consequently they gained the impression that Ahuramazda was greater than Zarathustra, which was wrong.

Lord Meher, Original Publication, Bhau Kalchuri, Vol. 3, 803.