Lord God, whereby shall I know. It may appear absurd, first, that Abram, who before had placed confidence in the simple word of God, without moving any question concerning the promises given to him, should now dispute whether what he hears from the mouth of God be true or not. Secondly, that he ascribes but little honor to God, not merely by murmuring against him, when he speaks, but by requiring some additional pledge to be given him. Further, whence arises the knowledge which belongs to faith, but from the word? Therefore Abram in vain desires to be assured of the future possession of the land, while he ceases to depend upon the word of God. I answer, the Lord sometimes concedes to his children, that they may freely express any objection which comes into their mind. For he does not act so strictly with them, as not to suffer himself to be questioned. Yea, the more certainly Abram was persuaded that God was true, and the more he was attached to His word, so much the more familiarly did he disburden his cares into God's bosom. To this may be added, that the protracted delay was no small obstacle to Abram's faith. For after God had held him in suspense through a great part of his life, now when he was worn down with age, and had nothing before his eyes but death and the grave, God anew declares that he shall be lord of the land. He does not, however, reject, on account of its difficulty, what might have appeared to him incredible, but brings before God the anxiety by which he is inwardly oppressed. And therefore his questioning with God is rather a proof of faith, than a sign of incredulity. The wicked, because their minds are entangled with various conflicting thoughts, do not in any way receive the promises, but the pious, who feel the impediments in their flesh, endeavor to remove them, lest they should obstruct the way to God's word; and they seek a remedy for those evils of which they are conscious. It is, nevertheless, to be observed, that there were some special impulses in the saints of old, which it would not now be lawful to draw into a precedent. For though Hezekiah and Gideon required certain miracles, this is not a reason why the same thing should be attempted by us in the present day; let it suffice us to seek for such confirmation only as the Lord himself according to his own pleasure, shall judge most eligible.

And he said, Lord God - אדני יהוה Adonai Yehovah, my Lord Jehovah. Adonai is the word which the Jews in reading always substitute for Jehovah, as they count it impious to pronounce this name. Adonai signifies my director, basis, supporter, prop, or stay; and scarcely a more appropriate name can be given to that God who is the framer and director of every righteous word and action; the basis or foundation on which every rational hope rests; the supporter of the souls and bodies of men, as well as of the universe in general; the prop and stay of the weak and fainting, and the buttress that shores up the building, which otherwise must necessarily fall. This word often occurs in the Hebrew Bible, and is rendered in our translation Lord; the same term by which the word Jehovah is expressed: but to distinguish between the two, and to show the reader when the original is יהוה Yehovah, and when אדני Adonai, the first is always put in capitals, Lord, the latter in plain Roman characters, Lord. For the word Jehovah see on Genesis 2:4 (note), and on Exodus 34:6 (note).
Whereby shall I know - By what sign shall I be assured, that I shall inherit this land? It appears that he expected some sign, and that on such occasions one was ordinarily given.

And he said, Lord GOD, (b) whereby shall I know that I shall inherit it?
(b) This is a particular motion of God's Spirit, which is not lawful for all to follow, in asking signs: but was permitted for some by a peculiar motion, as to Gideon and Ezekiel.

And he said, Lord God, whereby shall I know that I shall inherit it? Not as questioning or doubting whether he should or not; but this he asked for the further confirmation of his faith in the promise, and for the sake of his posterity, that they might more easily and strongly believe that they should inherit the land given and promised to them; nor is it culpable to ask a sign of God with such a view; good men have done it, as Gideon, Judges 6:36, and Hezekiah, 2-Kings 20:8, without being blamed for it; yea, Ahaz is blamed for not asking one, Isaiah 7:10.

Whereby shall I know that I shall inherit it? - This did not proceed from distrust of God's power or promise, but he desired this, For the strengthening of his own faith. He believed, Genesis 15:6, but here he prays, Lord help me against my unbelief, Now, he believed, but he desired a sign, to be treasured up against an hour of temptation. For the ratifying of the promise to his posterity, that they also might believe it.