V.14 God had promised a son to Abraham about 15 years before Isaac was born when Abraham and Sarah were about 100 years old! Isaac was heir of all which Abraham had and had been promised by God. Galatians 4:1-9 tells us that we who have received the Lord Jesus as our Savior are heirs. Notice that we are “an heir of God.”
V.19-21 Abraham could not wait for God to give him and Sarah a son, so he had married Sarah’s Egyptian servant whose name was Hagar. Now this son could not share with Isaac all the privileges of being the heir of Abraham. Sarah acts unkindly and drives away the servant Hagar and her boy Ishmael. God was allowing this (v. 12), because He had a lesson to teach us. Before we try to learn what all this story means, it is wonderful to see that God took care of the woman and her son (verses 15-21). God is merciful. Now the meaning of the story (Read Gal. 4:22-31). The question is “is the believer under law?” The child of the bondwoman (Hagar) is a picture of the people who are under the law. The Galatians 4:24 has the word “allegory,” this means a story, a parable, a type. That which actually happened, and then what it is a picture of. So Hagar and Ishmael are a picture of the law. Sarah and Isaac are a picture of grace (or promise). We believers are not saved by keeping the law, but by grace (Eph. 2:8; Gal. 5:4; Rom. 5:14).
V.22-34 Abimelech, a Gentile, sees that God is with Abraham, and looks up to him and wants to make an agreement with him. He looks to Abraham for protection. You may find this part hard to understand. It is a look into the future a picture of the millennium (Rev. 22:2, last part).