Hebrews 6:12, 17

so that you will not be [spiritually] sluggish, but [will instead be] imitators of those who through faith [lean on God with absolute trust and confidence in Him and in His power] and by patient endurance [even when suffering] are [now] inheriting the promises.

In the same way God, in His desire to show to the heirs of the promise the unchangeable nature of His purpose, intervened and guaranteed it with an oath