“How he went into the house of God in the days of Abiathar the high priest, and did eat the shewbread, which is not lawful to eat but for the priests, and gave also to them which were with him?”
In the days of Abiathar the high priest, he entered the house of God and ate the consecrated bread, which is lawful only for priests to eat. And he also gave some to his companions — Mark's text says Abiathar while 1 Samuel 21 names Ahimelech, creating a historical detail that has generated significant discussion. The most likely explanation is that Abiathar (the son who survived Saul's massacre of the priests and became the famous high priest under David) was so closely associated with his father's priesthood that Mark uses his better-known name. The point of the David precedent is clear: even the most sacred restrictions can yield to genuine need, and the scribal tradition itself recognizes this in the way it treats the Davidic story.
Church Fathers on Mark 2:26
We read also in the following part, that they who came and went away were many, and that they had not time enough to take their food, wherefore, according to man’s nature, they were hungry.
For it was a precept in Israel, delivered by a written law, that no one should detain a thief found in his fields, unless he tried to take something away with him. For the man who had touched nothing else but what he had eaten they were commanded to allow to go away free and unpunished. Wherefore the Jews accused our Lord’s disciples, who were plucking the ears of corn, of breaking the sabbath, rather than of theft.
For David, when flying from the face of Saul [1 Sam 21] went to the Chief Priest, and ate the shew-bread, and took away the sword of Goliath, which things had been offered to the Lord. But a question has been raised how the Evangelist called Abiathar at this time High Priest, when the Book of Kings calls him Abimelech.