It is extremely important to make a distinction between the old and the new covenants. We tend to think of just the old and new testament, which is essentially the same thing. But, we must understand, this is why Paul was so strong in saying we (including the Gentile Church), are not under the law, but under a promise, one of God's eternal Grace.
Tell me, you who desire to be under the law, do you not listen to the law? For it is written that Abraham had two sons, one by a slave woman and one by a free woman. But the son of the slave was born according to the flesh, while the son of the free woman was born through promise. Now this may be interpreted allegorically: these women are two covenants. One is from Mount Sinai, bearing children for slavery; she is Hagar. Now Hagar is Mount Sinai in Arabia; she corresponds to the present Jerusalem, for she is in slavery with her children. But the Jerusalem above is free, and she is our mother. For it is written,
“Rejoice, O barren one who does not bear;
break forth and cry aloud, you who are not in labor!
For the children of the desolate one will be more
than those of the one who has a husband.”
Now you, brothers, like Isaac, are children of promise. But just as at that time he who was born according to the flesh persecuted him who was born according to the Spirit, so also it is now. But what does the Scripture say? “Cast out the slave woman and her son, for the son of the slave woman shall not inherit with the son of the free woman.” So, brothers, we are not children of the slave but of the free woman.
Type and shadow
Israel under the law, before Christ died once for all, was "son" of the slave woman under the curse of the law. This is the old convenant of the law. Not free!
The Church is made free!
The Church, like Isaac, is of a promise, the new covenant and will inherit.
Study of connections
And if you are Christ's, then you are Abraham's offspring, heirs according to promise.
This means that it is not the children of the flesh who are the children of God, but the children of the promise are counted as offspring. For this is what the promise said: “About this time next year I will return, and Sarah shall have a son.”
Acts 3:25 - Peter points out the Abrahamic and Isaac covenant of promise from the beginning of the Church
You are the sons of the prophets and of the covenant that God made with your fathers, saying to Abraham, ‘And in your offspring shall all the families of the earth be blessed.’
- https://www.biblegateway.com/passage/?search=Galatians+4%3A21-31&version=ESV - Apostle Paul's analogy
- https://www.openbible.info/labs/cross-references/search?q=Galatians+4%3A28 - Cross references
Unless otherwise indicated, all Scripture quotations are from The ESV® Bible (The Holy Bible, English Standard Version®), copyright © 2001 by Crossway, a publishing ministry of Good News Publishers. Used by permission. All rights reserved.