The Birth of Isaac, Genesis 21:1-7
We can only imagine the joy that Abraham and Sarah felt at the birth of their son, Isaac. They had been wandering in the land which God had promised to Abraham for twenty-five years. Abraham was one hundred years old, and Sarah, who was ninety, had been unable to conceive children. Now, finally, the son whom God had promised them was here! Sadly, it is not a joy that is to remain so absolute; consequences of Abraham and Sarah's earlier sins would soon bring them pain.
Isaac and Ishmael, Genesis 21:8-21
No woman likes to see her child being mocked or bullied, but Sarah's natural desire to protect her son Isaac was no doubt intensified by the long wait, and the many years of thinking she would never have a child to call her own. So, we can understand her grief when she saw Ishmael, who was 14 years older than Isaac, treating her son badly. It must have brought back painful memories of the circumstances surrounding the elder brother's conception, Genesis 16:1-6. However, her insistence that Hagar and Ishmael be cast out was more than a mere emotional outburst: it was confirmed by God Himself, in 21:12. We are told of Abraham's love for Ishmael. Even though he knew that God had promised to care for his first son, it must have been very difficult for Abraham to send the boy and his mother away with nothing but bread and water (keep in mind, Abraham was a tremendously wealthy man, 13:2). Imagine Abraham's regret over the sin he had committed with Ishmael's mother, Hagar. Make no mistake: actions have consequences. Nothing we do escapes God's notice, Galatians 6:7.
Now, we may wonder, why would God instruct Abraham in this way? We know that God did indeed take care of Hagar and Ishmael, yet nonetheless it seems harsh to us that He would have wealthy Abraham send them out with nothing. Well, as it turns out, God had more in mind than these five people. He had in view His ultimate plan for dealing with sin, and He knew what His plan entailed. Hagar and Sarah come to represent the covenant with Moses at Sinai, and the covenant in Jesus' blood. Our lesson is that those who hold to the law and covenant delivered through Moses will have no share in the inheritance that is in Christ, as the inspired apostle Paul teaches us in Galatians 4:21-31. Those who look for some future establishment of the physical city of Jerusalem in the physical nation of Israel as a sign of God's concern for them, are missing the mark. The Israelites are no more God's people than any other physical nation of people. All people of all nations must come to God through faith in Jesus Christ: Romans 3:21-30; John 14:6.
The Promises to Isaac, Genesis 26:1-6
The promises to Abraham are continued through Isaac: the land, the innumerable descendants, and the fact that they would be a blessing to all nations. This time, again, the promises are directly connected with Abraham's faithful obedience to God, verse 5.