Abiathar - (Easton's Bible Dictionary)
Father of abundance, or my father excels, the son of Ahimelech the high priest. He was the tenth high priest, and the fourth in descent from Eli. When his father was slain with the priests of Nob, he escaped, and bearing with him the ephod, he joined David, who was then in the cave of Adullam (1 Sam. 22:20-23; 23:6). He remained with David, and became priest of the party of which he was the leader (1 Sam. 30:7). When David ascended the throne of Judah, Abiathar was appointed high priest (1 Chr. 15:11; 1 Kings 2:26) and the "king's companion" (1 Chr. 27:34). Meanwhile Zadok, of the house of Eleazar, had been made high priest. These appointments continued in force till the end of David's reign (1 Kings 4:4). Abiathar was deposed (the sole historical instance of the deposition of a high priest) and banished to his home at Anathoth by Solomon, because he took part in the attempt to raise Adonijah to the throne. The priesthood thus passed from the house of Ithamar (1 Sam. 2:30-36; 1 Kings 1:19; 2:26, 27). Zadok now became sole high priest. In Mark 2:26, reference is made to an occurrence in "the days of Abiathar the high priest." But from 1 Sam. 22, we learn explicitly that this event took place when Ahimelech, the father of Abiathar, was high priest. The apparent discrepancy is satisfactorily explained by interpreting the words in Mark as referring to the life-time of Abiathar, and not to the term of his holding the office of high priest. It is not implied in Mark that he was actual high priest at the time referred to. Others, however, think that the loaves belonged to Abiathar, who was at that time (Lev. 24:9) a priest, and that he either himself gave them to David, or persuaded his father to give them.