Genesis 15-18

One of the things that I love about the Bible is that it is really real. Real people, with real struggles, trying to be faithful to this great God we serve. In Abram & Sarai we see a couple willing to leave everything they've known in order to follow God, and yet they struggled with God's promise for a son. They were old, well beyond normal child bearing age, the promise of a child seemed more difficult for them to believe than believing that God would deliver the promised land. As we read the text we do not see them question the land, we see them laugh at the idea of finally becoming parents. Of course, the promise of land was to come through their promised son, so if God did not give that son their decedents would never inherit any land because there would be no decedents.

But God is set on them being parents, He does not get frustrated and give up when they laugh at this great promise, He doesn't even turn away from them when they foolishly sought to accomplish His work through carnal means (Genesis 16). Rather, He continues to affirm His promise to them. After Abram went into Hagar and she bore him a son God got so specific that Abram would be unable to misunderstand, even if he struggled to believe.

And God said, Sarah thy wife shall bear thee a son indeed; and thou shalt call his name Isaac: and I will establish my covenant with him for an everlasting covenant, and with his seed after him. (Gen 17:19)

Despite this seemingly impossible act, (now called) Abraham sought to honor God in other ways (being hospitable to strangers for example), and despite Abraham and (now) Sarah's initial questioning/ unbelief God had purposed that it would be through these two that His Christ would eventually come. God sought to bless Abraham and Sarah, and through that faithfulness He worked His perfect will and blessed millions of people over the span of centuries.