Everyone who reads Paul knows that Abram believed God and it was counted to him as righteousness (Genesis 15:6). What is not evident in Paul, but certainly assumed by him, is the specific promise that Abram believed: “So shall your descendants be” (Genesis 15:5).
How shall Abram’s descendants be? Yahweh offers a visual display of His verbal promise: “Look toward the heavens and recount the stars, if you are able to recount them” (v. 5).
That’s about numbers, of course: There are innumerable stars, and there shall be innumerable descendants of Abram. One pauses to wonder whether those who insist most vociferously on justification by faith really believe this promise: Do they believe that the seed of Abram will be as numerous as stars? If not, how can they share the faith of Abram? It’s ironic that some who profess justification by faith have trouble believing the promise the belief in which justifies Abram.
Abram is justified by faith in Yahweh’s promise, but Yahweh’s is an astronomical promise. Abram is justified by trusting the message of the stars: Justification by astronomy.