BROTHERS and SISTERS, when God made a promise to Abraham, since He could swear by no one greater, He swore by Himself, saying, “I will surely bless you and multiply you.” Thus Abraham, having patiently endured, obtained the promise. People, of course, swear by someone greater than themselves, and an oath given as confirmation settles all disputes. In the same way, when God desired to show even more clearly to the heirs of the promise the unchangeable nature of His purpose, He guaranteed it by an oath, so that through two unchangeable things, a promise and an oath, in which it is impossible for God to lie, we may have a strong encouragement, since we have taken refuge and seized the hope set before us.We have this hope as a sure and steadfast anchor of the soul, a hope that enters the inner shrine beyond the veil, where Jesus has entered as a forerunner on our behalf, having become a high priest forever according to the order of Melchizedek.
Saint John Chrysostom says that both the promise and the patience are important,
Do you see that God's promise alone did not effect the whole, but Abraham's patient waiting as well? This shows that oftentimes, a promise is thwarted through faintheartedness.