Genesis 21 is the story of Isaac's miraculous birth and the sending away of Ishmael and his mom, Hagar. Casting them out might seem cold and harsh but it is absolutely crucial for us to do. Paul in Galatians 4 allegorizes this story. He says it stands for a spiritual reality going on inside us. Isaac and Ishmael are in us. They are fighting. You know what I mean. Just read Romans 7.
Before you're saved you were fairly content. You didn't think about your soul that much. After you're saved it seems you have a battle. The old nature while defeated is still there and raises up its ugly head. We can overcome it. Ephesians 4: 22-24: " You were taught, with regard to your former way of life, to put off your old self, which is being corrupted by its deceitful desires: to be made new in the attitude of your minds: and to put on the new self, created to be like God in true righteousness and holiness."   

When you read Gen. 20 you think, haven't I read this same story before? And the answer would be yes you have, Gen 12:10-20. It was Egypt that time, it's Gerar this time. Abraham tells Sarah to say that he's her brother and he'd say she's his sister. He did it to protect himself. Each time it put Sarah in danger and damaged his reputation. At Gerar, it caused women to become barren, almost cost the king his life and put into question who the father of soon to be born Isaac. This was a bad sin and Abraham keeps on repeating it.

Don't we? What is your besetting sin? That moral weakness in your character that you repeat over and over? As I came to Gen 20 I was surprised at what I found. I found a God who went out of His way to protect Abraham from the harm that could have been the result of his sin.  Abraham and Sarah both were  blessed financially, God used Abraham's prayers to heal the king and his harem, and God forgave him.

If you love God and confess your sin, it doesn't have to mean its over. God is a merciful God. Romans 2:4- ...not realizing that God's kindness leads you toward repentance.