The main point in Genesis 21:1-7 is not the miraculous birth of Isaac. Its main point is how close of a relationship God provides for Abraham and Sarah. This is clear when reading carefully verses 1 and 2, where in three different ways (visited, promised, spoken), the witter is telling us of God’s personal care and involvement in his people’s life. In a way, he is also telling us to look back, to what God has said and promise He would do, to see the root of God’s behavior, i.e. His character.