17. Wherein--that is, Which being the case among men, God, in accommodation to their manner of confirming covenants, superadded to His sure word His oath: the "TWO immutable things" (Heb 6:18).
      willing . . . counsel--Greek, "willing . . . will"; words akin. Expressing the utmost benignity [BENGEL].
      more abundantly--than had He not sworn. His word would have been amply enough; but, to make assurance doubly sure, He "interposed with an oath" (so the Greek). Literally, He acted as Mediator, coming between Himself and us; as if He were less, while He swears, than Himself by whom He swears (for the less among men usually swear by the greater). Dost thou not yet believe, thou that hearest the promise? [BENGEL].
      heirs of promise--not only Abraham's literal, but also his spiritual, seed (Ga 3:29).

JFB.