Homilies on the Epistle to the Hebrews

God, however, ought to have been believed even without an oath: “wherein” (he says) God willing more abundantly to show unto the heirs of promise the immutability of His counsel, confirmed it [lit. “mediated”] by an oath. In these words he comprehends also the believers, and therefore mentions this “promise” which was made to us in common [with them]. “He mediated” (he says) “by an oath.” Here again he says that the Son was mediator between men and God.