In 1 Samuel 13, Saul is king and yet performs priestly duties even though he shouldn't have. Samuel rebukes Saul and tells him that his kingdom is slowly coming to an end because of his disobedience. ...
In the old testament, there are three (maybe more?) references to a priesthood. Genesis 14:18 Melchizedek king of Salem brought out bread and wine. (Now he was the priest of the Most High God.) 19 ...