John Lightfoot on Mark 2:26

[In the days of Abiathar the high priest.] It is well enough known what is here said in defence of the purity of the text; namely, that Ahimelech the father was called Abiathar, and Abiathar the son was called also Ahimelech. But I suppose that something more was propounded by our Saviour in these words. For it was common to the Jews under Abiathar to understand the Urim and Thummim. Nor without good reason, when it appears, that under the father and the son, both of that name, the mention of inquiring by Urim and Thummim is more frequent than it is ever anywhere else; and, after Abiathar the son, there is scarcely mention of it at all. Christ therefore very properly adds, in the days of Abiathar the high priest, therein speaking according to a very received opinion in the nation: as though he had said, "David ate the shewbread given him by the high priest, who had the oracle by Urim and thummim present with him, and who acted by the divine direction."

"Ahitophel, that is, a counsellor, Benaiah, the son of Jehoiada, that is, the Sanhedrim; Abiathar, that is, Urim and Thummim."