Galatians 4:28-31

Now you, brothers and sisters, like Isaac, are children of promise. At that time the son born according to the flesh persecuted the son born by the power of the Spirit. It is the same now. But what does Scripture say: "Get rid of the slave woman and her son, for the slave woman's son will never share in the inheritance with the free woman's son." (Genesis 21:10) Therefore, brothers and sisters, we are not children of the slave woman, but of the free woman. 

Now you, brothers and sisters, like Isaac, are children of promise 
Galatians 4:23  His son by the slave woman was born according to the flesh, but his son by the free woman was born as the result of a divine promise.
Galatians 3:16  The promises were spoken to Abraham and to his seed. Scripture does not say "and to seeds," meaning many people, but "and to your seed," meaning one person, who is Christ.

At that time the son born according to the flesh persecuted the son born by the power of the Spirit 
Genesis 21:9  But Sarah saw that the son whom Hagar the Egyptian had borne to Abraham was mocking, ...

"Get rid of the slave woman and her son, for the slave woman's son will never share in the inheritance with the free woman's son." 
Genesis 21:10  ... and she said to Abraham, "Get rid of that slave woman and her son, for that woman's son will never share in the inheritance with my son Isaac."

we are not children of the slave woman 
Romans 7:4  So, my brothers and sisters, you also died to the law through the body of Christ, that you might belong to another, to him who was raised from the dead, in order that we might bear fruit for God. 

but of the free woman 
Galatians 4:22  For it is written that Abraham had two sons, one by the slave woman and the other by the free woman. 

(parentheses are mine)

Scriptures taken from the Holy Bible, New International Version® NIV®, Copyright© 1973, 1978, 1984, 2011 by Biblica, Inc.® Used by permission. All rights reserved worldwide. 
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