Hebrews 6:13

13.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. (105) 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.