Hebrews 6:18

With this hope, then, let us attach ourselves to the one who is faithful to promises and just in judgments. The one who bids us to refrain from lying is all the less likely to lie. For nothing is impossible to God, save lying.

Through Moses God gave commandments about sacrifices, and the whole book of Leviticus is taken up with acceptable ways for them to be carried out. The Lord, through the prophets, found fault with those who contemptuously misstated these things, calling them disobedient to the commandment. He told them, “I have not asked you to do these things!… Neither did I speak to your fathers about sacrifices, nor give them commands about whole burnt offerings.”Some have put forth the opinion that either the Scriptures do not agree or that God, who gave the commandment, is a liar. But in this there can be no disagreement—far from it. The Father, who is truth, cannot lie, “for it is impossible for God to lie,” as Paul affirms. Actually, these things are plain to those who accept the writings of the law with faith and look at them in the right way. Here is my explanation, and may God grant by your prayers that I am not too far from the truth. It does not appear to me that God gave the commandments and the law concerning sacrifices right away when he led them out of Egypt. Nor did he who gave the law really pay any attention to the whole burnt offerings, as such. He was looking ahead to those things that were prefigured and pointed out by them. “For the law has but a shadow of the good things to come.” And “Those regulations were set forth until the time of reformation.”9
That is why the whole law did not deal with sacrifices, although it did include commands concerning them. By means of these commands it began to teach people, calling them away from idols and drawing them to God, giving them proper teaching for the times in which they lived. So you see, God did not give the people those commands about sacrifices and offerings when he brought them out of Egypt, nor even when they first came to Mount Sinai. God is not like people, that he should want those things for himself. No, he gave the commandment so that they might know him and his Word (the Son)—and forget about those so-called gods that do not really exist but appear to do so because of the show people put on.

“Through two unchangeable things” … the former is that he swore by himself. The latter is that David said, “The Lord has sworn and will not change his mind, that you are a priest forever after the order of Melchizedek.” It is by this means that we who have been made coheirs of his promise “might have strong encouragement.” We “have fled for refuge” in order to protect ourselves, not for God’s justice, in order that God may draw and drive us away from the evils of this world, and may open for us the way “into the inner shrine behind the curtain.” We do not go in first. We do not go into the shrine of the tabernacle, where Moses went, but into the inner shrine in heaven, “where Jesus has gone as a forerunner, having become a high priest forever,” not in order to offer the victims of sacrifices, like Aaron, but to offer the word for all nations, like Melchizedek.

“Through this” oath “God desired to show more convincingly to the heirs of the promise” that God’s promise, because indeed it is God’s, will never be changed. God’s oath was infallible in its being interposed, that is, between God, the angel and Abraham.

"That by two immutable things, in which it was impossible that God should lie." What are these two? The speaking and promising; and the adding an oath to the promise. For since among men that which is confirmed by an oath is thought more worthy of credit, on this account He added that also.

Seest thou that He regardeth not His own dignity, but how He may persuade men, and endures to have unworthy things said concerning Himself. That is He wishes to impart full assurance. And in the case of Abraham indeed the Apostle shows that the whole was of God, not of his patient endurance, since He was even willing to add an oath, for He by whom men swear, by Him also God "sware," that is "by Himself." They indeed as by one greater, but He not as by one greater. And yet He did it. For it is not the same thing for man to swear by himself, as for God. For man has no power over himself. Thou seest then that this is said not more for Abraham than for ourselves: "that we" (he says) "might have strong consolation, who have fled for refuge to lay hold on the hope set before us." Here too again, "after he had patiently endured he obtained the promise."

"Now" he means, and he did not say "when He swore." But what the oath is, he showed, by speaking of swearing by a greater. But since the race of men is hard of belief, He condescends to the same things with ourselves. As then for our sake He swears, although it be unworthy of Him that He should not be believed, so also did the Apostle make that other statement "He learned from the things which He suffered," because men think the going through experience more worthy of reliance.

What is "the hope set before us"? From these past events (he says) we conjecture the future. For if these came to pass after so long a time, so certainly the others will. So that the things which happened in regard to Abraham give us confidence also concerning the things to come.

When he said above, “the full assurance of the hope,” he made clear that God furnished to Abraham “full assurance,” not that which belonged to his conduct through works but that which belonged to him through God’s redemptive plan.

Paul added that “Christ” has become “an eternal high priest” for us, in that Christ leads all those believing through him in each generation to God based on the hope of the resurrection.

“Through two unchangeable things,” namely, that God would never be able to lie about what had been promised and that he made the promise with an oath.

By “curtain” he referred to heaven: the Lord promised to give the kingdom of heaven to those believing in him.

He augmented their confidence with the name forerunner: if he is our forerunner and has gone up for us, we too must follow and be granted ascent.

As God has no one superior to him (he is saying), his oath was witness; he swore by himself, in fact. Yet though making a promise and doing it with an oath, he did not immediately fulfill the promise; instead, the patriarch needed great patience, and only with the passage of a great length of time did he thus see the realization of the promise.

He is a high priest forever, not in offering sacrifices (having offered his own body once), but in being a mediator leading the believers to the Father; through him (he says, remember) we both have had access to the Father. The Lord himself says in the sacred Gospels, “No one comes to the Father except through me.” We must be aware, of course, that the divine apostle made mention of the oaths sworn to Abraham so that the unchanging character of the divine will should be brought out; it endures in advance the stability of high priesthood according to the order of Melchizedek, the oath being linked to the word at this place.

It was necessary, therefore, he says, to believe in God who had promised without an oath, but since God wanted to do something more, he mediated the promise by an oath.
"to show the firmness of His counsel." He also came to us. For we are the heirs of the promise, being the seed of Abraham; and if we are the seed, then we are also heirs.
“He mediated with an oath.” See what he prepares. That the Son was thereafter the mediator of God and men (for he has also become a mediator in the incarnation); and then he mediated with the Father and with Abraham, confirming him with an oath; for the Word was he who both spoke to Abraham and swore. The meaning of the interpretation is well-founded, so that it may be thus. God, that is, the Word, wanting to show the unchangeable nature of those glorifying the Father, mediated with the Father and with Abraham with an oath, that is, he became a mediator in the oath. For through him, as the Word, God and the Father swore.
"that by two unchangeable things." Two things, both that God has promised, and that He has added an oath to the promise. For since among men the oath seems more trustworthy, on account of this He also added it Himself.
"In which it is impossible." In which, instead of "from which," from these two things, the promise is shown to be most trustworthy, and it is impossible for God to lie. Therefore, just as He swore by us (although it is unworthy of Him to swear), so also understand the "from which" He learned what He suffered. For men also consider it more credible to come through experience.
"we might have a strong encouragement," That is, a great exhortation and urging.
"who have fled." To him, he says. "to hold,” let us have strong encouragementto lay hold to the hope set before us. From those, he says, that were said to Abraham, let us be faithful also to the things to come, and to the kingdom of heaven. For this is our hope, which now is set before us in expectation, but then will be realized, as it was said; From those things, this will be understood. There it was made true, that in these things we may believe. And that for this reason he swore then, that now we may believe concerning the things to come. For there was nothing else, he says, and the things then promised with oaths, or the future hope of the faithful.
"we have as an anchor," which stabilizes the wavering ships, and hope makes those who are shaken by temptations brave, committed, and patient.
"safe and committed." Not simply an anchor, but also safe and secure. For there is an anchor that does not keep the ship unshaken when it is smaller than it ought to be.
"which enters." In order that, he says, while we are still here, hope may enter into the kingdom of heaven, that is, so that we may already be in our hopes. See what hope does. It anticipates what is to come.
“enters into the inner sanctuary behind the curtain.” The inner part of the curtain is called heaven.
“where Jesus has entered as a forerunner on our behalf.” Having said that our hope enters into heaven, he confirms what has been said, trusting this by the facts. For Christ himself entered there; and not only did he enter, but he entered on our behalf. And where our forerunner has entered, there it is necessary for us also to enter. For the forerunner is the forerunner of those who follow; and the difference between the forerunner and those who follow is not very great, just as between John and Christ. Do not be disturbed, then; as long as we have not yet entered, we will enter where our forerunner is. The "on our behalf" is extended so that he may be called our forerunner. The fact that he entered in the flesh is on our behalf, so that we also, having flesh, may obtain entrance.
“Having become a high priest.” And this is a great encouragement, if indeed our high priest is above and much better than those among the Jews; first, in manner; for he is not according to their order, but according to the order of Melchizedek; then, in place and tabernacle; for he is above and in heaven; third, in the covenant; for it is upon greater and more perfect promises; likewise, in permanence; for he is eternal, and not temporary; lastly, in person; for he is the Son of God. Therefore, do not lose heart. And well is the term "having become," because it was according to the flesh that he became a high priest.