The Background
We don´t know for sure how far back it all started, but probably there always existed a priesthood. It is not wrong to assume that at this early period there were special persons chosen to honour the deites at their shrines. From the early beginning this might have been the clan leader or community leader, or at least someone whose capabilities appeared as unexplainable to the group. During the Early Dynastic Period at least, it was already the king who carried the main responsibility of acting as the intermediate between men and gods. This was soon partly delegated to the priesthood whose task it was to care for the most important citizens of Egypt, i.e. the gods and so the average Egyptian could live safe in the knowledge that everything was taken care of, that the gods were treated with the utmost attention and therefore stayed on in the land, thereby keeping it safe and prosperous.

The King - The High Priest
It is important to note, however, that the King remained the highest priest in the country thorughout the historic periods and that the priesthood served as his deputies, and many times at his whim. No matter how 'called' a person might feel to work in temples (after all, it entailed good chances for making a career, and it put food on the table and a roof over one´s head,) the King could do as he pleased, and substitute priests without forewarning and elevate others. To be of good family and linked by blood to the King also had its advantage.

Temlpe Organisation
Government and temple was closely linked and from earliest times, the priestly offices were often held by members of the royal family, since it had been the head of the clan or group, who acted as an intermediary between the deties and humans. This developed into the king being considered as divine and soon the theocratic state developed. In the Middle Kingdom civil administrators like town mayors acted as high priests. This was still the case in the New Kingdom. The office of governor or mayor of the city and the overseer of godservants were one and the same until the reign of Hatshepsut.

The larger temple centers were like veritable cities and employed all kinds of workers. There were temple bakeries and breweries, and some temples had a fleet of boats and fishermen, hunters and stables with donkeys and, in time horses. There were farms or tenants of land. The produce arrived daily to fill the god's storehouses and everything was recorded and kept in books by the temple scribes. On top of that the temple employed craftsmen like stonemasons, painters, carpenters, weavers, scribes and administrators, not to mention the priests. It was a huge organization and as an example there is Karnak, with a list of more than 81.000 employees!

The Priesthood
There were several levels of priesthood. They varied throughout the different time periods as the priesthood developed and grew, and as more cult centers were built and competed for power. They also varied between the cult centra but it seems that at all places there was also a cader of women working in the temples, probably as dancers, singers and musicians. As time passed, the priesthood grew and many other functions became attached to temples, like overseers of the different levels of priesthood, of distribution of offerings, of goods and deliverance of food, of maintenance and enlarging of the temple buildings etc.

Few of the ancient priesthood worked full-time in the temples; this was common only in the higher hierarchy. Otherwise they kept on with family life and other occupations. Pharaoh was the highest priest and in fact a living god, who could perform rituals at any and all temples. He appointed High Priests who acted as his deputies.

During the time of service in the temple, any employee enjoyed a higher standard than normally. The god´s wealth was for all to share, though depending of status and type of work. So large, well functioning centras, usually held large properties of land and the workers, stone masons, farmers etc. at these large temle estates kept their families there too, which enlarged the population, and ensured there were all these extra kinds of needs and doings to keep women and children fed and clothed. A large religious temple complex was bested in riches only by the Royal House.

Being in the priesthood meant for many a safe way of living, the temples received offerings and were landholders. To that came the religious experience, which certainly must have been profound for those working with rituals close to the cult statue.

Sources:
Temples of Ancient Egypt - edited by Byron Shafer
The Priests of Ancient Egypt - Serge Sauneron
Jan Assman; ‘State and Religion in the New Kingdom’.