John Chrysostom

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 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.