There are stories, legends, mythologies, folklore, traditions, histories, scriptures, and texts from all around the world which tell us of humans who have reached the stature of the gods. They have, in essence, become "a god." Some recognizable examples are people such as Jesus, Siddhartha Gautama (Buddha), and Muhammad. The process that these went through has been called theosis, deification, divinization, realization, awakening, and enlightenment. What does it mean to become or be "a god"?
I perceive that priesthood was originally an inner experience of Divine Power and Presence in humans, a deep Realization when they subdued their egoic psychological "natural man" self in consciousness, and perceived their real Being underneath it in Glory, beyond words. I believe humans eventually organized this mystical experience, and the knowledge of it, instituting it into the "ordination" to an "office" of "priesthood," meant to indicate those who had direct experience of God.