The Rise of the Priests
” … Corresponding to the growth of the temples, we find the organization of the cult extending its scope; and with this extension, the steadily increasing power and authority of the priests. In the small beginnings of the Euphratean cities, the priestly and secular functions no doubt rested in one and the same person.
The ruler of a city or district, as we have seen,  was regarded as the representative of the deity. As such he stood in a special relation to the deity, acting as a mediator between the latter and the people, while upon his good standing with the god, the general welfare of the people depended. On the very ancient monument of Ur-Nina  we find the ruler himself offering the libation to the god, though behind him stands an attendant who is probably a priest to assist in carrying out the rite.
As early, however, as the days of Gudea (c. 2450 B.C.) the ruler himself is led into the presence of the deity through the mediation of a priest. Gudea is so depicted on seal cylinders and other monuments, and presumably therefore the marked differentiation between priest and ruler thus illustrated was at the time an established custom of long standing.
The mediatorship may, indeed, be set down as the chief prerogative of the priest in Babylonia and Assyria. With this as a starting-point, his other functions as sacrificer, as exerciser, as inspector of the liver for the purpose of ascertaining the disposition of the deity, as astrologer and as diviner in general, interpreting birth-signs, dreams, and furnishing the answer as to the meaning of all kinds of occurrences that deviated from the normal or that in any way aroused attention, may be derived.
The people could proceed as far as the inner court of the temples, where an altar stood, but beyond that the priests alone could venture, and the rulers only if accompanied by a priest who as the privileged servitor of the deity had access to the divine presence.
Intercession is thus a distinguishing function of the priest, as a corollary to his role as mediator.”
Morris Jastrow, The Civilization of Babylonia and Assyria, 1915, pp. 271-2.