A.T. Robertson 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)

The house of God (τον οικον του θεου). The tent or tabernacle at Nob, not the temple in Jerusalem built by Solomon.

When Abiathar was high priest (επ Αβιαθαρ αρχιερεως). Neat Greek idiom, in the time of Abiathar as high priest. There was confusion in the Massoretic text and in the LXX about the difference between Ahimelech (Abimelech) and Abiathar (2 Samuel 8:17), Ahimelech's son and successor (1 Samuel 21:2; 1 Samuel 22:20). Apparently Ahimelech, not Abiathar was high priest at this time. It is possible that both father and son bore both names (1 Samuel 22:20; 2 Samuel 8:17; 1 Chronicles 18:16), Abiathar mentioned though both involved. Επ may so mean in the passage about Abiathar. Or we may leave it unexplained. They had the most elaborate rules for the preparation of the shewbread (τους αρτους της προθεσεως), the loaves of presentation, the loaves of the face or presence of God. It was renewed on the commencement of the sabbath and the old bread deposited on the golden table in the porch of the Sanctuary. This old bread was eaten by the priests as they came and went. This is what David ate.