And you brethren, like Isaac, are children of promise. Galatians 4:28

The Apostle Paul is using the example of Hagar and Sarah that there are symbolically two covenants. One covenant is the Mosaic Law which keeps everyone under the yoke and in slavery. The other covenant is Christ which sets us free from the law under grace.

This is allegorically speaking, for these women are two covenants: one proceeding from Mount Sinai bearing children who are to be slaves; she is Hagar. Galatians 4:24

Now notice one of these covenants (Sarah) was the promise from God, while the other covenant (Hagar) was from the works of man. God promised Abraham to deliver him the Son Isaac, and he fulfilled that promise. But while Abraham became impatient , he had another son with Hagar, that was not part of Gods plan. One covenant came from God, while the other covenant came from the works of man

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

Those who are under the covenant of the law are persecuting those under the law of grace. Why? Because one Covenant is of Christ, while the other covenant is against Christ. The Holy Spirit is now within them, so they have been circumcised of the heart

For God so loved the world, that He gave His only begotten Son, that whoever believes in Him shall not perish, but have eternal life. John 3:16

God delivered us his promised Son, so we can become Children of God. Jesus is the Messiah, the Promised One throughout the scriptures. Whoever believes in Jesus shall not perish Amen!