Hebrews 6:17

Wherein God, willing more abundantly to shew unto the heirs of promise the immutability of his counsel, confirmed it by an oath:

Wherein - i:e., Which being the case among men, God, in accommodation to their manner of confirming covenants, added to His word His oath: the "TWO immutable things" (Hebrews 6:18).

Willing ... counsel, [ Boulomenos (G1014) ... boulees (G1012)] - 'willing ... will: the utmost benignity.

More abundantly - than had He not sworn. His word would have been amply enough; but, to make assurance doubly sure, He [ emesiteusen (G3315) horkoo (G3727)] 'interposed as Mediator with an oath,' coming between Himself and us: as if He were less while He swears, than Himself by whom He swears (for the less usually swear by the greater). Dost thou not yet believe, thou that hearest the promise? (Bengel.)

Heirs of (the) promise - not only Abraham's literal, but also his spiritual, seed (Galatians 3:10).