"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 offered to us may be greatly encouraged" (Hebrews 6:16-18)
Even in today’s society an oath is sacred. To take an oath is to swear that what has been promised is true, binding all parties by this vow. We even have notaries who mark their seal on the oath certifying the vow. In the legal system willingly giving false information when making an affidavit is the crime of perjury. In other words, making an oath is sacred and guarantees the oath taker's honesty and integrity in the matter under question.
When God made the promise to Abraham that he would have a son, make him into a great nation and the Messiah would come through his line, since there was no one greater for God to swear by, he swore by himself on oath (Hebrews 6:13). He vowed the promise to Abraham would come to pass. He did this to show the unchanging nature of the promise. Meaning every blessing would be fulfilled and the promise would never be revoked. Even today we live under the blessings of this vow made to Abraham.
Amazingly both of these things, the promise and the oath, will always exist together because they are tied to God’s nature and integrity. Just like water, both hydrogen and oxygen must be present, otherwise it is just elements, not substance. The oath made to Abraham makes it impossible for God to lie, prove false or deceive us. This is because the oath rests on the character of God. In fact since the world was spoken into existence by Jesus and it is held together by His word of power (Genesis 1; John 1:1-3; Colossians 1:16-17; Hebrews 1:2-3). If God lied, the world would cease to exist because the Word holds all things together. If God were ever proved false, His Word would be broken and there would be no power to sustain the universe. Which is why he confirmed his promise to Abraham with an oath. The stakes were incredibly high and he wanted to show the unchanging nature of the promise. What He said would come to pass.
We can draw strong encouragement from this since we too live under the blessing promised to Abraham. Everything God has said, has come to pass. Even in the midst of the trials of this world we have a hope that is unparalleled by any other. This hope is an anchor for the soul and it is guaranteed by both the promise made and the oath taken (Hebrews 6:19-20). In the midst of life we can rest securely knowing that our future is secured, our hope sure and our God faithful.