1 Samuel 22:20 - Albert Barnes' Notes on the Bible

Abiathar - He may have remained at Nob to take care of the sanctuary when the other priests went to Saul, and so escaped. He continued David’s faithful friend throughout his reign 1Sa 23:9; 1 Samuel 30:7; 2 Samuel 15:24, 2Sa 15:29, 2 Samuel 15:35, but gave offence by taking Adonijah’s part against Solomon 1Ki 1:7, 1 Kings 1:19, 1 Kings 1:42, and in consequence was deprived of the high priesthood 1 Kings 2:26-27. In Mark 2:26, he is spoken of as the High Priest who gave the showbread to David. Perhaps he was the instigator of this act of kindness to David; and for this cause, as well as his constancy to David, is mentioned by our Lord instead of Ahimelech. It is also possible that, as sagan to his father, he may have performed most of the priestly functions, as Hophni and Phinehas did in the lifetime of Eli. Abiathar did not actually join David until he went to Keilah (marginal reference).