But as at that time he who was born according to the flesh persecuted him who was born according to the Spirit, so it is now also. 

                                                                                                            Galatians 4:29 

It’s as great approach to religious legalism than I can imagine. Paul’s combination of clarity and frankness drives home the point. 

Tell me, you who want to be under law, do you not listen to the law? 

                                                                                                Galatians 4:21

Why would anyone want to be under the Law? Perhaps works-based religion gives the participant a sense of control over their spiritual life. A life of faith in Jesus requires trust and surrender. A life of works does not. So even though voluntarily placing oneself under the Law is unimaginable, Paul challenges these individuals to consider the Law. His point is that the Law only makes demands and is incapable of bringing true spiritual freedom. He uses old covenant history to drive home this truth.

For it is written that Abraham had two sons, one by the bondwoman and one by the free woman. But the son by the bondwoman was born according to the flesh, and the son by the free woman through the promise. This is allegorically speaking, for these women are two covenants…

                                                                                                            Galatians 4:22-23 

In order to understand the circumstances behind the birth of Ishmael and Isaac, take some time and read Genesis 15-18. For our purposes, we will examine the allegory Paul uses to draw a distinction between the old and new covenants. The first covenant is represented by the birth of Ishmael. This is the child that Abraham had with Hagar, who was not his wife, rather a maidservant. Consider the description of this covenant:

Born to the bondwoman

Born according to the flesh

Proceeding from Mount Sinai (The giving of the Ten Commandments)

Bearing children who are to be slaves

Represents the old covenant God made with Israel

The descriptors for this covenant are self-explanatory. The Law demands self-effort and rule keeping. It does not promote spiritual life rather it brings death. The second covenant is represented by the birth of Isaac. This is the promised child born to Abraham and Sarah in their old age. By default this covenant is the opposite:

Born to the free woman

Born according to promise

Proceeding from the cross of Jesus Christ

Bearing children who are to be free

Represents the new covenant God has made with all mankind (Jews and Gentiles)

Paul reminds the believers in Galatia that they were not part of the bondwoman (The Law) rather they were children of promise and of the free woman (Galatians 4:31). For us the message is simple, God, by His grace, has given us Jesus. His performance on the cross makes a way for sinners to be saved. As we receive salvation by faith in Jesus, we are no longer under the curse of the Law. We are redeemed and set free to live an abundant, God-honoring life. Condemnation is gone for those who are in Christ (Romans 8:1). The old covenant is now obsolete and is powerless to save anyone. Attempting to gain God’s favor by works is an offense to the cross of Jesus and is a spiritual dead end. This truth is a stumbling block for the religious, but for sinners, it is music to their ears.

Such confidence we have through Christ toward God. Not that we are adequate in ourselves to consider anything as coming from ourselves, but our adequacy is from God, who also made us adequate as servants of a new covenant, not of the letter but of the Spirit; for the letter kills, but the Spirit gives life.

                                                                                                         2 Corinthians 3:4-6