St. Paul finds the deeper meaning under the account of Sarah and Hagar. He is not denying the natural meaning of the history. It is true that Sarah was Abraham’s wife, and Hagar was her maidservant. It is true that God promised that the Seed would come through Abraham (Gen 12:1ff).
Hagar was thrown out with Ishmael because Ishmael was trying to put himself over Isaac (Gen 21:8ff). All of the history is true, but Paul finds a deeper lesson hidden under the history. He is not saying, necessarily, that Hagar and Ishmael fell away from faith. In fact, there is strong evidence that they were in fact restored (Gen 21:17ff). Rather, Paul is drawing from certain details of Moses’ account, which confirm certain spiritual truths, such as the freedom we have in Christ.
This is called the mystical sense of Scripture. It is not another meaning among other possible meanings in the text. Rather, it is nothing other than the gospel, which is plainly taught in God’s promises to Abraham. We learn from these promises that God’s inheritance is not one gained by human merits, but through faith, just as Abraham believed and was declared righteous (Gen 15:6). This is clear from the plain meaning of Moses’ words.
We, therefore, find that when Sarah says that the slave cannot have any share in the inheritance of the free, her plain words are in fact confirming the other deeper truth, which God gave to Abraham. Her words confirm how much less our slavish works share in what is freely given by our heavenly Father. No matter how much Ishmael tried he would never be the true heir. How much less can our works inherit eternal life!
Isaac did not earn his inheritance but was born free. So even less are we born by our own works, but rather by the water and the Spirit of freedom (John 3:5) through the resurrection of Jesus Christ from the dead (1 Pet 1:3; 3:21). We are nourished in the care of the church who is free from sin and all ills because of her Bridegroom, Christ. So just as Moses’ account confirms the truth of this spiritual freedom, may it be confirmed in our lives as well!