In this way God, desiring more abundantly to show the heirs of the promise the unchangeable nature of His own purpose, confirmed it by an oath; so that by two immutable things, in which it was impossible for God to lie, we who have fled for refuge might have strong encouragement to lay hold on the hope that has been set before us . . .(Hebrews 6:17 - 18).
These two things involve the true hope that only the true Church of God has. The writer of Hebrews takes the next two chapters to detail out these two immutable things.
Notice that Hebrews 7 deals with the greatness of the Melchisedek priesthood as opposed to the Aaronic priesthood. The hope for Israel was to be a great physical nation that would lead all nations to keep God’s commandments so they would have great physical blessings.
This is the hope of most every human today. This hope had a physical priesthood to teach God’s laws and offer sacrifices for occasional sins.
A better hope
But God’s oath was to bring in a better hope - the awesome hope of eternal life as a member of the God family inheriting the earth. This hope is one of the immutable things God says he will accomplish in spiritual Israel.
I will put my laws into their mind, and write them in their hearts; and I will be to them a God and they will be to me a people (Hebrews 8:10).
David was a man after God’s own heart. God will make us all after His heart. When God gets us through this school we call life, we will be able to say to sin temptation "Been there, done that!" Notice we will be God’s people, God’s own kind of being.
I will dwell in them and walk in them; and I will be their God, and they shall be My people . . . And I shall be a Father to you, and you shall be My sons and daughters (2Corinthians 6:16, 18).
But what is the other immutable thing and oath required to accomplish the hope?
But He was made a Priest with the swearing of an oath by Him Who says concerning Him, "The Lord swore and will not revoke His word, 'You are a Priest forever according to the order of Melchisedec'"). By such a greater measure then, Jesus was made the Guarantor of a superior covenant (Hebrews 7:21 - 22).
The word of the oath makes the son who is consecrated forevermore (see Hebrews 7:28).
So God has sworn two things. The first is to make us (mankind) into his very kind of people, in his likeness and image. The second is that he has sworn that Jesus will do this through his office of High Priest. He will deliver us from having eaten of the tree of the knowledge of good and evil, which tree makes us like God (Genesis 3:22) but gave us death.