Genesis 26:3
Sojourn in this land, and I will be with thee, and will bless thee; for unto thee, and unto thy seed, I will give all these countries, and I will perform the oath which I sware unto Abraham thy father; Sojourn in this land, and I will be with thee, and will bless thee; for unto thee, and unto thy seed, I will give all these countries, and I will perform the oath which I sware unto Abraham thy father;
To Isaac was renewed the promise first sworn to Abraham, that divine presence would attend him, blessing would rest upon him, and all these lands would be given to him and to his seed, with the oath to Abraham performed. That renewal was linked to Abraham's obedience to the divine voice and commands. Jacob later recalled this promise made to Isaac as ground for hope that special blessing would come upon Joseph.
In the words to Isaac God swore by himself to perform the oath sworn to Abraham his father, showing that God himself takes an oath. Such divine swearing proves that solemn appeal to God is right. Therefore because God swears, the Christian who lives in the light of the divine countenance and grows strong in divine strength can consistently testify under oath and appeal to God when matters of importance must be decided by law.