Jamieson, Fausset and Brown on Hebrews 6:13

13. For--confirming the reasonableness of resting on "the promises" as infallibly sure, resting as they do on God's oath, by the instance of Abraham. "He now gives consolation, by the oath of God's grace, to those whom, in the second, third, and fourth chapters, he had warned by the oath of God's 'wrath.' The oath of wrath did not primarily extend its force beyond the wilderness; but the oath of grace is in force for ever" [Bengel].