This verb appears in Hebrews 6:17, describing how God, wanting to show the unchangeable nature of his purpose to the heirs of promise, 'guaranteed it with an oath.' The word denotes acting as a mediator or surety, standing between parties to ensure an agreement. What's remarkable here is that God himself acts as the guarantor of his own promise — he mediates his own covenant. Since he could swear by no one greater, he swore by himself, making his promise doubly certain through both word and oath, providing strong encouragement for those who flee to take hold of the hope set before them.