Albert Barnes Commentary Mark 2:26

"How he entered into the house of God when Abiathar was high priest, and ate the showbread, which it is not lawful to eat save for the priests, and gave also to them that were with him?" — Mark 2:26 (ASV)

Abiathar the priest. From 1 Samuel 21:1, it appears that Ahimelech was high priest at the time referred to here. And from 1 Samuel 23:6, it appears that Abiathar was the son of Ahimelech. Some difficulty has arisen in reconciling these accounts. The probable reason Mark says it was in the days of Abiathar is that Abiathar was better known than Ahimelech.

The son of the high priest was regarded as his successor and was often associated with him in the duties of his office. It was not improper, therefore, to designate him as high priest, even during his father's life, especially as that was the name by which he was later known.

Abiathar, moreover, in the calamitous times when David came to the throne, left Saul's side and fled to David, bringing with him the ephod, one of the distinctive garments of the high priest. For a long time during David's reign, he was high priest, and it therefore became natural to associate his name with David's; to speak of David as king, and Abiathar as the high priest of his time.

This accounts for why he was spoken of, rather than his father. At the same time, it was strictly true that this occurred in the days of Abiathar, who was later high priest and was commonly spoken of as such. For example, we say that General Washington was present at Braddock's defeat and saved his army, though the title of general did not belong to him until many years later.

Shewbread. See Barnes on Matthew 12:4.