"When God made his promise to Abraham, since there was no one greater for him to swear by, he swore by himself, saying, 'I will surely bless you and give you many descendants.' And so after waiting patiently, Abraham received what was promised." NIV translation
We all are familiar with the practice of swearing by something greater than oneself to prove its truth. What do you do if you are the Creator of all things and there is nothing greater? That is how it was and is with God and His promises. There is no one greater for Him to swear by so the promise is simply based on His character and the fact that He keeps His promises. God is the same today as He has always been and so we can have that same patient assurance that what He has promised us will be received.
"Men swear by someone greater than themselves, and the oath confirms what is said and puts an end to all argument. Because God wanted to make the unchanging nature of his purpose very clear to the heirs of what was promised, he confirmed it with an oath. God did this so that, by two unchangeable things in which it is impossible for God to lie, we who have fled to take hold of the hope that is offered to us may be greatly encouraged." NIV translation
God made this oath to give the children of Abraham the assurance of truth. Abraham held onto the truth of God by faith and it is easier for us to hold on as we see that God kept this oath to him.
"We have this hope as an anchor for the soul, firm and secure. It enters the inner sanctuary behind the curtain, where Jesus, who went before us, has entered on our behalf. He has become a high priest forever, in the order of Melchizedek." NIV translation
Our hope is in Christ as our High Priest and it is sure as we see that "He has become a high priest forever". We do not have to worry about doing something that will cause us to lose our salvation. If our soul holds onto that like an anchor for a ship, we will not be tossed about by all the latest teachings and religious theories. We truly will have a blessed assurance!