Galatians 4:27-28 NASB
27 For it is written,
"REJOICE BARREN WOMAN WHO DOES NOT BEAR;
BREAK FORTH AND SHOUT YOU WHO ARE NOT IN LABOR;
FOR MORE ARE THE CHILDREN OF THE DESOLATE
THAN OF THE ONE WHO HAS A HUSBAND."
28 And you brethren, like Isaac, are children of promise.

In the NASB, a quote all in caps indicates that it came from the Old Testament. Using the cross references for Galatians 4:27, we find that this quote came from Isaiah 54:1.

Isaiah 54:1 NASB
"Shout for joy, O barren one, you who have borne no child;
Break forth into joyful shouting and cry aloud, you who have not travailed;
For the sons of the desolate one will be more numerous
Than the sons of the married woman," says the LORD.

If you were to continue reading chapter 54, you would soon realize that the barren one to whom the Lord is speaking is Israel. Paul has taken that verse and is now applying it in this situation with Hagar and Sarah.
Who was the barren woman for a long time--Hagar or Sarah? See Genesis 11:30, 15:2, 16:1, and 17:17.
What promise did God make to Abraham concerning Sarah? See Genesis 17:15-19 and 15:4-6.
Who was Abraham and Sarah's child?
In verse 28, how does Paul refer to the Galatians, his brethren?
Whom are they like in this regard?
So, what could you call Isaac?
Isaac's birth was made possible by God's intervention, which God had promised to Abraham.
How are the sons of a married woman made possible?
Once again, Paul is making a contrast. What is that contrast?
According to this quote, how should we react to knowing that God intervenes and brings forth numerous children of promise?
In telling the allegory about the two women and in giving this quote, what does Paul hope to accomplish in the lives of the Galatians?