John Calvin Commentary Genesis 15:8

"And he said, O Lord Jehovah, whereby shall I know that I shall inherit it?" — Genesis 15:8 (ASV)

Lord God, whereby shall I know? It may appear absurd:

1. That Abram, who before had placed confidence in the simple word of God without raising any question concerning the promises given to him, should now dispute whether what he hears from the mouth of God is true or not.
2. That he ascribes little honor to God, not merely by murmuring against Him when He speaks, but by requiring some additional pledge to be given him.

Furthermore, from what source does the knowledge that belongs to faith arise, if not from the word? Therefore, Abram desires in vain to be assured of the future possession of the land if he ceases to depend upon the word of God.

I answer that the Lord sometimes allows His children to freely express any objection that comes into their mind. For He does not act so strictly with them as to prevent Himself from being questioned. Indeed, the more certainly Abram was persuaded that God was true, and the more he was attached to His word, the more familiarly he unloaded his cares into God’s bosom.

To this may be added that the protracted delay was a significant 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 again 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. Therefore, his questioning with God is a proof of faith rather than a sign of unbelief. The wicked, because their minds are entangled with various conflicting thoughts, do not receive the promises in any way. But the pious, who feel the impediments in their flesh, endeavor to remove them so that they do not obstruct the way to God’s word, and they seek a remedy for those failings of which they are conscious.

Nevertheless, it is to be observed that there were some special impulses in the saints of old, which we should not now take as a precedent. For though Hezekiah and Gideon required certain miracles, this is not a reason why we should attempt the same thing today. It should be enough for us to seek only such confirmation as the Lord Himself, according to His own pleasure, deems most appropriate.