In these 12 verses, Paul explains the promise God made to Abraham which was not dependent on what Abraham had done or had not done but on the grace of God. God chose Abraham to be the 'father of the nations' because God chose him. The Abrahamic covenant is solely dependent on God's decision to put everything together. Abraham's responsibility was just to trust and obey. Paul stresses in this passage that, God's ability to keep His promise is not dependent on the faithfulness of man nor is it dependent on the law, but it is dependent on His faithfulness alone.
Paul acknowledges that Abraham and Sarah in themselves were both too old, weak, and frail to have children. Furthermore, Sarah was barren and unable to have children due to her age. But God used this circumstance for His glory to show that His grace and power are both infinite and sufficient. Through the divine use of this couple, God showed that faith is man putting trust in what God can do, not God putting trust in what man can do.
Like Abraham and Sarah, we bring many mistakes, failures, and baggage to the table but God removes those mistakes and decides to bless us despite our shortcomings. God promises hope when all seems hopeless and provides a way when there seems to be none. Our response is simply to trust and obey. We will never do enough, make enough, or believe enough to earn God's favor, we need to recognize that God is the provider, Jehovah Jireh. Our response is simply to trust and obey, for there is no other way, to be happy in Jesus than to trust and obey!