Fausset's Bible Dictionary
("father of abundance".) The only son of Ahimelech, the high priest, who escaped the slaughter committed by Saul at Nob, on Doeg's information that Ahimelech had inquired of the Lord for David, and given him the shewbread and the sword of Goliath (1 Samuel 22). Eighty-five persons wearing the priestly linen ephod were killed. Abiathar, with an ephod (the high priest's mystic scarf) in his hand, escaped to David. It is an instance of God's retributive justice that Saul's murder of the priests deprived him thenceforth of their services in inquiring of the Lord (1 Chronicles 13:3); step by step he sank, until, bereft of legitimate means of obtaining divine counsel, he resorted to the illicit course of consulting the witch of Endor, and so filled the measure of his iniquity and brought on himself destruction (1 Chronicles 10:13). David, on the contrary, by sheltering Abiathar was enabled to inquire of the Lord in the ordained way (1 Samuel 23:6-9; 1 Samuel 30:7; 2 Samuel 2:1; 2 Samuel 5:19; 2 Samuel 21:1, an undesigned coincidence with Psalms 16:7, and so a proof of genuineness).
Abiathar adhered to David during all his wanderings, and was afflicted in all wherein David was afflicted; also when he assumed the throne in Hebron, the Aaronite priestly city of refuge. He bore the ark before David when it was brought up from Obed-Edom's house to Jerusalem (1 Chronicles 15:11-12; 1 Kings 2:26). He was loyal in Absalom's rebellion; and, subordinate to Altithophel, was the king's counselor (1 Chronicles 27:34). But in Adonijah's attempt to be David's successor, instead of Solomon, Abiathar, probably from jealousy of Zadok, who was on Solomon's side, took Adonijah's part. David had evidently for some time previous given the first place in his confidence to Zadok, a preference the more galling as Abiathar was the high priest and Zadok only his vicar, or sagan; thus it was to Zadok he gave the command to take the ark back in Absalom's rebellion. Abiathar is mentioned subordinately 1 Samuel 15:25; 1 Samuel 15:29; 1 Samuel 15:35.
Perhaps Zadok was appointed high priest by Saul after the slaughter of Ahimelech. David on succeeding, to conciliate his subjects, allowed him conjointly to hold office with Abiathar. Zadok had joined David in Hebron after Saul's death, with 22 captains of his father's house (1 Chronicles 12:28). Abiathar had the first place, with the ephod, Urim and Thummim, and the ark, in the tent pitched by David at Jerusalem Zadok officiated before the tabernacle and brazen altar made by Moses and Bezaleel in the wilderness, which were now in Gibeon (1 Chronicles 16:1-7; 1 Chronicles 16:37; 1 Chronicles 16:39-40; 1 Chronicles 27:38; 1 Chronicles 27:34; 2 Chronicles 1:3-5). Moreover, Zadok and Abiathar represented rival houses: Zadok that of Eleazar, the oldest son of Aaron; Abiathar that of Ithamar, the youngest (1 Chronicles 24:3-4; 1 Chronicles 6:8). Eli, of whose family it had been foretold 150 years before that the priesthood should pass from it, was Abiathar's progenitor fourth backward, and Abiathar would naturally fear the coming realization of the curse. All these undesigned proprieties mark the truth of the history. His own act brought the prophecy to its consummation (1 Samuel 2:31-35). Solomon banished him to Anathoth, and put Zadok as high priest in his room (1 Kings 2:35). But in 1 Kings 4:4 Abiathar is still called the "priest" second to Zadok. The Septuagint, "the king made Zadok the first priest in the room of Abiathar," solves the difficulty. Abiathar had been first, priest, but henceforth he was made subordinate to Zadok. Ahimelech or Abimelech, son of Ahimelech, is substituted for Ahimelech, son of Ahimelech: 2 Samuel 8:17; 1 Chronicles 18:16; 1 Chronicles 24:3; 1 Chronicles 24:6; 1 Chronicles 24:31. The Lord Jesus (Mark 2:26) names Ahimelech as the high priest in whose time David ate the shewbread. Probably the sense is: "in the days of Ahimelech, who was afterward high priest," and under whom the record of the fact would be made. Perhaps too the loaves being his perquisite (Leviticus 24:9) were actually handed by Ahimelech to David. Both father and son, moreover, it seems from the quotations above, bore both names, and were indifferently called by either.
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Fausset, Andrew R. Entry for 'Abiathar'. Fausset's Bible Dictionary. https://www.studylight.org/dictionaries/eng/fbd/a/abiathar.html. 1949.