The first men, says Porphyry, sacrificed only plants.  For such a simple ritual, each man could be a pontiff in his family.
The natural desire to please the deity multiplied ceremonies, because of which the men, occupied with agriculture, became incapable of performing them all and fulfilling the details.
Particular places were dedicated to the gods ; there had to be ministers to take care of them, as each citizen takes care of his house and domestic affaires. Thus the peoples who have no priests are ordinarily barbarians. Such the Pedalians once were,  and such are still the Wolgusky. 
Persons dedicated to the deity had to be honored, especially among peoples who had come up with a certain notion of a corporal purity requisite for approaching the places most agreeable to the gods, and dependent on certain practices.
The service of the gods requiring continual attention, most peoples were inclined to make a separate body of the clergy. Thus, among the Egyptians, the Jews, and the Persians,  certain families were consecrated to the deity, which were self-perpetuating and did the service. There were even some religions which had not only the idea of separating ecclesiastics from business, but also of sparing them the trouble of a family, and this is the practice of the principal branch of Christian law.
I shall not adress here the consequences of the law of celibacy : it is obvious that it could become harmful to the degree that the body of the clergy was too extensive, and that consequently the body of the laity would not be extensive enough.
By the nature of human understanding, in matters of religion we like whatever supposes an effort, as in morality we like speculatively whatever bears the connotation of severity. Celibacy has been more agreeable to peoples to whom it seemed the least suited, and for whom it could have more regrettable consequences. In the countries of southern Europe, where by the nature of the climate the law of celibacy is more difficult to observe, it has been retained ; in those of the north, where the passions are less lively, it has been banished. Furthermore, in countries where there are few inhabitants, it has been allowed ; in those where there are many, it has been rejected. It will be obvious that all these observations bear only on the excessive extension of celibacy, and not on celibacy as such.