For when God made a promise to Abraham, etc. His object was to prove, that the grace of God is offered to us in vain, except we receive the promise by faith, and constantly cherish it in the bosom of our heart. And he proves it by this argument, that when God promised a countless offspring to Abraham, it seemed a thing incredible; Sarah had been through life barren; both had reached a sterile old age, when they were nearer the grave than to a conjugal bed; there was no vigor to beget children, when Sarah's womb, which had been barren through the prime of life, was now become dead. Who could believe that a nation would proceed from them, equaling the stars in number, and like the sand of the sea? It was, indeed, contrary to all reason. Yet Abraham looked for this and feared no disappointment, because he relied on the Word of God. [1] We must, then, notice the circumstance as to time, that the Apostle's reasoning may appear evident; and what he subjoins refers to this -- that he was made partaker of this blessing, but that it was after he had waited for what no one could have thought would ever come to pass. In this way ought glory to be given to God; we must quietly hope for what he does not as yet show to our senses, but hides from us and for a long time defers, in order that our patience may be exercised. Why God did swear by himself we shall presently see. The manner of swearing, Except blessing I will bless thee, we have explained what it means in the third chapter: God's name is not here expressed, but must be understood, for except he performs what he promises, he testifies that he is not to he counted true and faithful.

Footnotes

1 - It is said, that having "patiently endured" or rather waited, "he obtained the promise," that is, of a numerous posterity, the particular thing previously referred to. After having waited for twenty-five years, (see Genesis 12:1-4, and Genesis 17:1-16,) a son was given him; and this beginning of the fulfilled promise was a pledge of its full accomplishment. This case is brought forward as an example of waiting faith. -- Ed.

For when God made promise to Abraham - That he would bless him, and multiply his seed as the stars of heaven; Genesis 22:16-17. The object of introducing this example here is, to encourage those to whom the apostle was writing to persevere in the Christian life, This he does by showing that God had given the highest possible assurance of his purpose to bless his people, by an oath. Reference is made to Abraham in this argument, probably, for two reasons:
(1) To show the nature of the evidence which Christians have that they will be saved, or the ground of encouragement - being the same as that made to Abraham, and depending, as in his case, on the promise of God; and,
(2) because the "example" of Abraham was just in point. He had persevered. He had relied firmly and solely on the promise of God. He did this when appearances were much against the fulfillment of the promise, and he thus showed the advantage of perseverance and fidelity in the cause of God.
Because he could swear by no greater - There is no being greater than God. In taking an oath among people it is always implied that the appeal is to one of superior power, who is able to punish for its infraction. But this could not occur in the case of God himself. There was no greater being than himself, and the oath, therefore, was by his own existence.
He sware by himself - Genesis 22:16. "By myself have I sworn;" compare Isaiah 45:23. In an oath of this kind God pledges his veracity; declares that the event shall be as certain as his existence; and secures it by all the perfections of his nature. The usual form of the oath is, "As I live, saith the Lord;" see Numbers 14:21, Numbers 14:28; Ezekiel 33:11.

When God made promise to Abraham - The promise referred to is that made to Abraham when he had offered his son Isaac on the altar, Genesis 22:16-18 : "By myself have I sworn, saith the Lord; for because thou hast done this thing, and hast not withheld thy son, thy only son; that in blessing I will bless thee, and in multiplying I will multiply thy seed as the stars of the heaven, and as the sand which is upon the seashore; and thy seed shall possess the gate of his enemies; and in thy seed shall all the nations of the earth be blessed." Of this promise the apostle only quotes a part, as is generally the case, because he knew that his readers were well acquainted with the Scriptures of the Old Testament, and particularly with the law.
He sware by himself - He pledged his eternal power and Godhead for the fulfillment of the promise; there was no being superior to himself to whom he could make appeal, or by whom he could be bound, therefore he appeals to and pledges his immutable truth and Godhead.

(7) For when God made promise to Abraham, because he could swear by no greater, he sware by himself,
(7) Another encouragement, to push them onward because the hope of the inheritance is certain, if we continue to the end, for God has not only promised it, but also promised it with an oath.

For when God made promise to Abraham,.... The apostle proposes Abraham as a pattern, because he was the father of these Hebrews, and of all believers; and because they were interested in the promise made to him, and had a right to the same blessing with him; and because he was remarkable for his faith and patience: the promise made to him is not that in Genesis 12:1 nor that in Genesis 15:1 but that in Genesis 22:16 for that only had an oath annexed to it: and this was made by Jesus Christ, there called the Angel of the Lord, and here God; and who is truly and properly so; and than whom there is none greater; and who elsewhere, as here, is said to swear by himself, Isaiah 45:23 as follows:
because he could swear by no greater, he sware by himself; swearing is ascribed to a divine person after the manner of men, and in condescension to them; and who is never introduced swearing, but in matters of moment and of great importance; the note of Philo the Jew (n) on the passage in Genesis 22:16 from whence the following words are cited, is worthy of observation, being very near the apostle's words;
"well does he (God) confirm the promise with an oath, and with an oath that becomes God; for you see that God does not swear by another, for nothing is better than himself, but by himself, who is the best of all; but some have suggested as if it was inconvenient to swear, for an oath is taken for the sake of faith; but God alone is faithful, &c.''
(n) Leg. Allegor. l. 2. p. 98.

For--confirming the reasonableness of resting on "the promises" as infallibly sure, resting as they do on God's oath, by the instance of Abraham. "He now gives consolation, by the oath of God's grace, to those whom, in the second, third, and fourth chapters, he had warned by the oath of God's 'wrath.' The oath of wrath did not primarily extend its force beyond the wilderness; but the oath of grace is in force for ever" [BENGEL].

For when God made promise. The promises are absolutely sure. See how God confirmed his promise to Abraham! See Genesis 22:15-18.
Saying, Surely blessing. The promise so confirmed to Abraham is given in these words.
He obtained the promise. It was sure, but he had to patiently endure in order to obtain. The history of Abraham shows how he was blessed.
For men verily swear by the greater. When men make a solemn oath in order to settle a fact or covenant beyond controversy they usually swear in the name of God. God, however, could swear by no one greater than himself.
God willing, etc. God, out of condescension to that human weakness which puts more confidence in an oath than in the bare word, confirmed his promise by an oath.
That by two immutable things. By his word of promise, and by his oath, neither of which could ever be broken.
Strong consolation. In the absolute certainty of God's promises who have fled for refuge. Fled from the wrath revealed against sin to the Gospel with its promises and blessed hopes.
Which hope . . . an anchor of the soul. As an anchor holds the ship when the storms are raging, so this hope holds the soul stedfast.
Entereth into that within the vail. The vail was before the Holy of Holies in the tabernacle, but it was a type of heaven. Hence this means that the hope reaches to heaven. It is a heavenly hope.
Whither the forerunner. As the High Priest entered within the veil into the Holy of Holies, so our High Priest has entered for us, and before us, into the heavens.
A High Priest forever, etc. See notes on Hebrews 7:1-10.

For - Ye have abundant encouragement, seeing no stronger promise could be made than that great promise which God made to Abraham, and in him to us.