Paul encouraged the Galatians to be faithful to the Gospel he had imparted to them, and to beware of Judaisers who had infiltrated the Church. These false teachers insisted on combining the Gospel of Grace with the Old Covenant Law.

Gal 4:22-26 KJV For it is written, that Abraham had two sons, the one by a bondmaid, the other by a freewoman. 23 But he who was of the bondwoman was born after the flesh; but he of the freewoman was by promise. 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. 25 For this Agar is mount Sinai in Arabia, and answereth to Jerusalem which now is, and is in bondage with her children. 26 But Jerusalem which is above is free, which is the mother of us all.

Paul explained that Hagar being a bondwoman represented the bondage of the Old Covenant and correlated to 'earthly Jerusalem', while Sarah a freewoman represented the freedom of the New Covenant and correlated to 'heavenly Jerusalem'.

The ‘Jerusalem which now is’ was in bondage to the Law given to Moses at Mount Sinai, whereas ‘Jerusalem which is above’ is New Covenant freedom in the Holy Spirit, through the death, resurrection and glorification of the Lord Jesus Christ.

Gal 4:30 KJV 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.

In a spiritual sense, Hagar the bondwoman typified the casting out of the Old Covenant, while Sarah the freewoman typified the establishing of the New Covenant.

The Church at Galatia had freedom, but was in danger of being rescinded if entangled in the bondage of Judaism and the Old Covenant Law.

Therefore, New Jerusalem is not a material city to descend from Heaven in the future, but a spiritual city to enjoy now in the New Covenant Age.