QUESTION:
What are the two immutable things of Hewbrews 6:13-20

Hebrews 6:31-20 (NASB)¹³ For when God made the promise to Abraham, since He could swear by no one greater, He swore by Himself, ¹⁴saying, “I WILL SURELY BLESS YOU AND I WILL SURELY MULTIPLY YOU.” ¹⁵And so, having patiently waited, he obtained the promise. ¹⁶For men swear by one greater than themselves, and with them an oath given as confirmation is an end of every dispute. ¹⁷In the same way God, desiring even more to show to the heirs of the promise the unchangeableness of His purpose, interposed with an oath, ¹⁸so that by two unchangeable things in which it is impossible for God to lie, we who have taken refuge would have strong encouragement to take hold of the hope set before us. ¹⁹This hope we have as an anchor of the soul, a hope both sure and steadfast and one which enters within the veil, ²⁰where Jesus has entered as a forerunner for us, having become a high priest forever according to the order of Melchizedek.

ANSWER:
The two immutable things are God’s promise and His oath. Neither one, is subject to change, and the promise alone would have been sufficient. But, God wanted to give strong encouragement to us, the heirs of the promise, and therefore confirmed it with an oath. The promise is given in Genesis 12 and the oath in Genesis 22. Wow, what assurance this gives to the believer! And, just to make the case as absolute as can be, as though to counter any wrong conceptions based on the earlier part of the chapter, we’re shown that we are already anchored in glory, where the Messiah Jesus has gone as a forerunner and ministers as our High Priest able to save to the uttermost the ones coming to glory through Him.

Hebrews 7:25 (NASB) ²⁵Therefore He is able also to save forever those who draw near to God through Him, since He always lives to make intercession for them.