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.

"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.

"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.

"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.

What kind and what kind? Both by the fact that He simply spoke and promised, and by the fact that He joined an oath to the promise. Since among people that which is confirmed with an oath is considered more trustworthy, God also joined one. "In which," instead of "from them" — from these two things the promise is most trustworthy, and "it is impossible for God to lie." Therefore, just as He swore for our sake, even though it is unworthy of Him to swear, so also understand that He "learned from the things which He suffered" (Heb. 5:8). For people also consider more trustworthy that which is known from experience.

That is, great consolation and encouragement. This is said not so much in relation to Abraham as in relation to us who have fled to Him, that is, who have placed our hope in Him. In what then do we have encouragement? In holding fast to "the hope set before us": that is, that we, on the basis of what was given to Abraham, would be convinced also regarding the promises concerning us, and would not doubt regarding the future and heavenly blessings for which we hope; on the contrary, that we would firmly and securely hold fast to this hope and not lose it. For the promise to Abraham is a promise to us as well, and preeminently to us who have believed in Christ, as was said above.

The second disposition for the inflowing of the gift of fortitude is through the imperturbable consolation of hope. Hence the Apostle to the Hebrews: "We have the strongest consolation, we who have fled to hold fast the hope set before us."

323. – Then (v.18) he shows the fruit of the promise: first, he shows what that fruit is; secondly, what follows upon it (v. 19).

324. – The fruit, of course, is that our hope is certain; hence, he says, that we might have strong, i.e., firm, encouragement of hope, by two unchangeable things, in which it is impossible that God should be false, namely, by God who promises and does not lie: 'God is not a man that he should lie' (Num. 23:19) and by the oath in which there is a stronger confirmation of the truth. But it should be noted that just as sensual delight is a sense experience and memory of the past, so hope is of the future: 'None that trust in him shall fail in strength' (1 Mac 2:61); 'They that hope in the Lord shall renew their strength, and they shall take wings as eagles; they shall run and not be weary, they shall walk and not faint' (Is. 40:31). We may have, I say, we who have fled for refuge from the evils of the world and the onslaught of the enemy, to seize the hope set before us: 'Lord, you have been my refuge' (Ps. 89:11); 'The name of the Lord is a strong tower; the just runs to it and shall be exalted' (Pr. 18:10).