Quick thought on Genesis 15
We see Abram's lack of faith right away, and his worry that the promise will end with him. But God promises a seed. This is a picture of how that our righteousness and our salvation are a gift of God and not of our own works. Abram and Sarai were as good as dead, and childbearing years had long since passed. But God promised that He would give Abram a son, and He did just that.