We rightly think of Abraham as our ancestor in faith, but his faith began small, just like all of ours. The faith necessary for God to count him righteous (Gen 15:6) was much less than the extraordinary faith demonstrated when he offered up Isaac years later (22:3). Abraham’s faith, like ours grew over the years. It was not something that he worked up by the strength of his will or by fertile imagination; it grew in response to witnessing God’s faithfulness over the years. He learned increasingly more deeply that God can be trusted, and he learned this because he had a relationship with God, where God spoke clearly and Abraham obeyed fully.
God had already promised Abram a seed (indeed, a “great nation,” 12:2a) and a land (12:1e). In 15:2, however, when God promises Abram a reward, Abram balks. What reward can count, since Abram is childless? He cannot pass on any of his blessings to his children; they will go instead to his leading servant (15:2-3). (In Mesopotamian custom, however, a designated heir could be displaced if the testator subsequently had a son.) Abram is not expecting a “great nation” to come from him, at least not genetically.
Yet God renews the promise, and affirms innumerable descendants for Abram (15:5). Keep in mind that Abram is now between 75 and 85 years old (12:4; 16:3, 16), Sarai is just ten years younger (17:17), and that they have had no children yet. (In fact, even when Abraham sees the promise fulfilled, there will be just one son of promise, who will also have just two sons himself.) Yet Abram believes that God will give him innumerable descendants (15:6a). This is significant faith, not unlike the act of faith Abram undertook when he left everything familiar to him in obedience to God’s call (12:4). Still, this faith has yet to be tested over time.
God counts this faith of Abram’s as righteousness (15:6); we obey (as in 12:4) because we believe God’s promise, so our most fundamental response to God, which he accepts, is trusting his Word, depending on what he says. But what immediately follows this beautiful expression of intimacy between God and Abram? God has assured Abram about his seed, and now assures Abram about the land; God’s plan is to give him the land to possess it (15:7). “How will I know that I will possess it?” Abram asks (15:8). This may not be doubting God’s word per se; he may simply wish to know how he can be sure that he will meet the conditions if the prophecy is conditional. But it seems that he is asking for a confirmation.
God grants him a visionary dream that promises him the land, although also showing the difficulties that his descendants will experience before that vision is fulfilled (15:9-16), essentially summarizing the first part of Exodus. God confirms this promise by entering into a clear covenant with Abram (15:17-21), even accommodating contemporary expectations for covenant sacrifices and meals. God comes down to Abram’s level to assure him.
But Sarai does not have children (16:1), and by Genesis’s chronology, she is roughly 75 years old (cf. 16:16; 17:17). Thus, following Mesopotamian custom among wealthy households, Sarai urges Abram to use Hagar, Sarai’s servant, as a sort of “surrogate mother” (as Renita Weems has put it) to bear a child in Sarai’s name (16:2-3). (As an Egyptian, Hagar was probably a servant given to Sarai and her family during their stay in Egypt; cf. 12:16, 20.) Childbearing and heirs were essential priorities in their milieu, and while God had promised Abram descendants, he had not specified that they would come physically through Sarai.
Why does the narrative about Hagar immediately follow God’s reaffirmation of his promises to Abram? Perhaps in part to show us the difficulty of understanding God’s purposes more fully even when he has spoken a few of the details to us; and perhaps also to illustrate that Abram’s faith in Gen 15:6 was just rudimentary faith, compared to the sort of faith Abram would exhibit in Gen 22.
The faith that it takes to be justified is rudimentary: we simply need to take God at his word that he has promised, namely, that he has provided us salvation by Jesus’s death and resurrection (Rom 4:22-25). That is, it is not difficult to exercise faith for salvation.
But as we continue to walk with God and persevere through tests of our faith, we grow to see that God is reliable and that even in the hardest times there is hope. Abraham had seen God provide him a son miraculously and believed that this same God would fulfill his promise if Abraham obeyed him fully (Heb 11:17-19). We may not always hear the details as clearly as Abraham did, but we have surely heard the message of the cross. God is trustworthy, and tests of our faith are opportunities for us to learn faith in a deeper way, beyond saving faith. We may learn, ever more deeply, that God is trustworthy; if we persevere, the hardest challenges to faith are the ones that ultimately drive home his faithfulness most deeply, because no matter what God still has a plan and purpose for us that lasts forever.