The theological argumentation and exhortation for endurance in Heb 6:13–20 involves a notion of divine simplicity in which God’s being is God’s own perfections. In Heb 6:13–20 the Lord’s self-oath from Gen 22:16 means that God is God’s own faithfulness, swearing an oath “by himself,” having “none greater by whom to swear” in order to guarantee an “unchangeable” promise to Abraham that has been eschatologically realized in the heavenly priesthood of the risen Messiah. The first part of this article sketches conversations past and present on the doctrine of divine simplicity, from historically influential traditions to recent proposals to retrieve, revise, or abandon simplicity. The second part develops a theological interpretation of Heb 6:13–20, comparing its reading of the Akedah with Philo of Alexandria’s On Allegorical Interpretation of Genesis 3.203–208. The third part indicates how a doctrine of simplicity informed by theological interpretation of Heb 6:13–20 contributes constructive resources for Christian hope.