In Galatians 4: 21-31 the Apostle Paul elaborates on the 2 covenants. One is said to be from Hagar, the bondwoman, and the other from Sarah, the freewoman. It would correspond to their sons respectively; Ishmael and Isaac, the two sons of Abraham.
God promised Abraham that from his seed would come the promised child, the one in whom all the earth would be blessed. This promise was fulfilled through his son Isaac, which was born to Sarah, the freewoman. This promise was fulfilled through the seed of Isaac which was of course Yeshua.
In these scriptures the Apostle Paul clearly delves into what the Jews refer to as PaRDeS, the four levels of scriptural understanding. He has left the first level of understanding called P'shat, the simple, direct and plain sense, literal meaning of the text and is digging into the deeper, sometimes allegorical understanding of scripture. He doesn't give a lot of explanation for his teaching but believes his readers understand the reference.
Allegorically, Hagar and Ishmael represent the flesh and sin covenant. Sarah and Abraham took matters into their own fleshly hands with their plan to produce a child with Hagar, Sarah's handmaiden. Hagar and Ishmael represent a rational relationship with God, one based on human nature and human understanding. Allegorically, Sarah and Isaac represent the spiritual/heavenly/non-sinful covenant or promise. Sarah and Isaac represent that which is of God, super natural and Supra-rational.
What most people fail to recognize is that two things were going on at Mt. Sinai. While Moses was on top of the mountain receiving the Torah from Yahweh, the children of Israel at the foot of Mt. Sinai were building the golden calf and calling it Yahweh, the god who brought them up out of Egypt.
During the first century, Jerusalem and her children had become corrupt. They had corrupted the way of Yahweh and His Torah by their traditions. They would be likened unto Hagar and Ishmael and what was going on at the bottom of Mt. Sinai. At the bottom of Mt. Sinai the people were taking matters into their own hands, like Sarah and Abraham with Hagar. They had grown tired of waiting on Moses to come down from the mountain and they decided to do the fleshly thing. They would build for themselves a god to worship, but as for Moses they didn't know what had happened to him. Paul likens his fellow brethren and leadership to the bondwoman and her son. They were of the fleshly covenant. They were of those things fleshly, not spiritual.
Those who serve Yahweh and His Torah the correct way, with fear and trembling, are likened to what was going on at the top of Mt. Sinai between Moses and Yahweh. These are of the freewoman, Sarah, and her son Isaac. The people who received Yeshua and His teachings, accepted His salvation through His shed blood, are of the heavenly Jerusalem. They obey God and seek only to hear His voice and the voice of the one whom He sent, Yeshua. They are of an everlasting covenant or promise. They partake of the promise made to Abraham, through his seed -Yeshua. They are the ones who will be able to inherit the incorruptible crown and partake of the Heavenly Jerusalem that is being prepared; where all things of the fleshly, sinful nature will have no place.
We must strive to be of the covenant of the free woman and forsake the fleshly/sinful; doing things our own way, covenant of the bond woman and her son.