Hebrews 6:17-18 reads as follows in the NASB:
[17] In the same way God, desiring even more to show to the heirs of the promise the unchangeableness of His purpose, interposed with an oath, [18] so 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.
What are the “two unchangeable things“ being referred to here?
The stock-standard answer to this age-old question is "the promise and the oath" as per Ellicott, Barnes, Poole, Meyer, Expositor's Greek, Cambridge, Bengel's Gnomen, Vincent, etc.
Let us examine Heb 6:18 more closely, but to do this we need to back up to v13 which I quote below (BSB):
When God made His promise to Abraham, since He had no one greater to swear by, He swore by Himself, saying, “I will surely bless you and multiply your descendants.” And so Abraham, after waiting patiently, obtained the promise.
Men swear by someone greater than themselves, and their oath serves as a confirmation to end all argument. So when God wanted to make the unchanging nature of His purpose very clear to the heirs of the promise, He guaranteed it with an oath. Thus by two unchangeable things in which it is impossible for God to lie, we who have fled to take hold of the hope set before us may be strongly encouraged.
Note that the oath to confirm the promise has validity because God does not change and His purpose does not change. Thus, the promise is confirmed by the oath which cannot change because that which it is sworn by is God Himself.
For God, when He made the promise to Abraham, since He could swear by no one greater, swore by Himself, saying, "Surely blessing, I will bless you; and multiplying, I will multiply you." And thus when Abraham had patiently endured, he obtained the promise. For men swear by the greater, and for all disputes among them an oath is final for confirmation. Therefore God, intending to show more abundantly to the heirs of the promise the unchangeableness of His counsel, interposed with an oath, in order that by two unchangeable things, in which it was impossible for God to lie, we may have strong encouragement, we who have fled for refuge to lay hold of the hope set before us,
God's promise and His oath
Taking into consideration the whole of Scripture, the two immutable things referred to in Hebrews 6:18 would be the truth or actuality of God Himself (there is none greater, vs. 13), and the verity of His Word (the immutability of His counsel, vs. 17, which never returns unto Him void, but accomplishes all whereto it is sent, Isaiah 55:11).
God's promise and his oath are not what IS immutable but they are what MAKE the two immutable things immutable. There are many things that God cannot do; God cannot deny himself, God cannot be tempted, God cannot change, God cannot break an oath, God cannot be defeated, God cannot resist a broken and a contrite heart... And again these things are what make the two unchangeable or immutable things immutable but they are NOT what IS immutable. The two immutable things are 'Jesus Christ a priest FOREVER after the order of Melchisedek (Psalms 110:4) and the RESURRECTION of Jesus (Psalms 2:7). Notice the words in those two references; "The Lord hath SWORN, and will not repent, Thou art a priest FOREVER after the order of Melchizedek." "I will declare the DECREE: the Lord hath said unto me, Thou art my Son; this day have I begotten thee. Paul explains clearly in Acts 13:33 what is meant by the word 'begotten'.
Oath and the promise of the Lord are two immutable things which never return void to all mighty God everlasting love to entire family of God himself
