Galatians 4:27

In Galatians 4:27 Paul quotes Isaiah 54:1 to teach a point about distinguishing between living under the law and living under faith in Christ's ability to completely save and keep the saved saved.

Verses 8-11 of Chapter 4 Identify who Paul is addressing this letter to - Christians who have been seduced into believing that it was necessary to keep the law to keep their salvation. They were truly saved but and were free from keeping the elements of the law but were willing placing themselves back under the bondage of the Law. The term brethren in verse 28 supports that these were Christians.

 Agar (Hagar) & Sarah - The Bond & Free - By Faith or By The Law
Galatians 4:21 ¶ Tell me, ye that desire to be under the law, do ye not hear the law?

Paul's question, " Do you perceive what the law is really teaching?.

22 For it is written, that Abraham had two sons, the one by a bondmaid, the other by a freewoman.

The bondwoman was Agar and Sarah, Abraham wife was the freewoman who was barren.

23 But he who was of the bondwoman was born after the flesh; but he of the freewoman was by promise.

Ishmael the son of Agar was conceived in a sinful act, after the flesh.  Isaac was the son of promise (Sarah's son)

24 Which things are an allegory: for these are the two covenants; the one from the mount Sinai, which gendereth to bondage, which is Agar.

Paul is using a historical story to teach a Biblical truth about two covenants made by God to the two sons of Abraham.

25 For this Agar is mount Sinai in Arabia, and answereth to Jerusalem which now is, and is in bondage with her children.

Agar is a picture of Mt. Sinai where the was given, Jerusalem was where the temple was and the Jewish leadership [scribes, Pharisees and priests] all three were closely (in their minds) guarded and enforced the Law. These Jewish leaders were intent on keeping the Jews under their thumb even after the Death, burial and resurrection of Christ. The Jerusalem of Paul's time was kept subject to the Law delivered at Sinai..

26 But Jerusalem which is above is free, which is the mother of us all.

The heavenly Jerusalem, is free from the bondage of the law (The Lamb is there at the right hand of the father) Holy Jerusalem is where the Son left to go to the cross - He is the one who makes you free from the law - John 8:36 If the Son therefore shall make you free, ye shall be free indeed..

27 For it is written, Rejoice, thou barren that bearest not; break forth and cry, thou that travailest not: for the desolate hath many more children than she which hath an husband.(Isaiah 54:1)
The barren was Sarah who had a husband - Agar was the desolate (sent a way to the desert), yet more 

Isaac had two sons (Esau and Jacob) Ishmael had 12 (Genesis 17:20) and the LORD promised Agar a great nation would come from her son. (See - Ge 16:10; 21:18)  God's chosen (the nation of Israel came from the barren who had a husband so she could sing a joyful song)

28 Now we, brethren, as Isaac was, are the children of promise.

The Messiah was promised to come from the line of Isaac. Isaac received the The promise of of John 6:36.

The Christian

29 But as then he that was born after the flesh persecuted him that was born after the Spirit, even so it is now.

The unsaved want the make the saved live as if they are not saved and place many sinful roadblocks to make sinning easy. Are there more prompting the Christian live a life of the saved or more trying o get the Christian to live as if they are unsaved.

30 Nevertheless what saith the scripture? Cast out the bondwoman and her son: for the son of the bondwoman shall not be heir with the son of the freewoman. (Genesis 21:10,12)

Just as The LORD told Abraham to put the bondwoman and her son out of his home because Ishmael was not going to have a part in the promises given to Abraham which were to be inherited by Isaac. The (the Galatian Christians were to cast out those who wanted to bring them back under the bondage of the law; we are to cast out of our lives those tempt us to live as if we are not saved. (2Corinthians 6:15)

31 So then, brethren, we are not children of the bondwoman, but of the free

Paul tells the Christians in Galatia they ars not under the bondage of the law any longed.

Note: Paul does not end 4:27 with "saith the LORD". LORD) all capital letters is used only with the nation of Israel. Paul was writing to mostly gentile Christians not the nation of Israel as Isaiah was.