Paul in this passage deals a death blow to the mindset that we can be saved by the law (good works). He says that the law is bondage. The law was never meant to get you into heaven, but to show us our need for a Savior (Galatians 3:23). The children of Hagar (the slave), are those who think they can get to heaven by keeping the law (good works) and will never be saved. Those of Sarah (the freewoman) are children of the promise of salvation by grace through faith (Ephesians 2:8-9). Paul said in Galatians 2:21 “I do not set aside the grace of God; for if righteousness comes through the law, then Christ died in vain.” Whose child are you?
“Many people talk about salvation by grace who do not seem to have the least conception of what grace is. They think that God gives them the grace to do the things that make them deserving of salvation. That is not it at all [cf. Rom. 3:24].” 2Ironside, p. 163.
Galatians 4:21-31 Tell me, you who desire to be under the law, do you not hear the law? 22 For it is written that Abraham had two sons: the one by a bondwoman, the other by a freewoman. 23 But he who was of the bondwoman was born according to the flesh, and he of the freewoman through promise, 24 which things are symbolic. For these are the two covenants: the one from Mount Sinai which gives birth to bondage, which is Hagar– 25 for this Hagar is Mount Sinai in Arabia, and corresponds to Jerusalem which now is, and is in bondage with her children– 26 but the Jerusalem above is free, which is the mother of us all. 27 For it is written: “Rejoice, O barren, You who do not bear! Break forth and shout, You who are not in labor! For the desolate has many more children Than she who has a husband.” 28 Now we, brethren, as Isaac was, are children of promise. 29 But, as he who was born according to the flesh then persecuted him who was born according to the Spirit, even so it is now. 30 Nevertheless what does the Scripture say? “Cast out the bondwoman and her son, for the son of the bondwoman shall not be heir with the son of the freewoman.” 31 So then, brethren, we are not children of the bondwoman but of the free.