Before the appointed time, the heir lived like a slave under guardians and trustees. Paul uses this to describe the pre-Christ condition under the elemental principles of the world.
Galatians 4:1-7
1 But I say that so long as the heir is a child, he is no different from a bondservant, though he is lord of all,
2 but is under guardians and stewards until the day appointed by the father.
3 So we also, when we were children, were held in bondage under the elemental principles of the world.
At the fullness of time, God sent his Son, born of a woman and born under the law, to redeem those under the law and secure adoption to sonship.
4 But when the fullness of the time came, God sent out his Son, born to a woman, born under the law,
5 that he might redeem those who were under the law, that we might receive the adoption of children.
Because believers are sons, God sent the Spirit of his Son into their hearts, crying 'Abba, Father,' confirming that they are no longer slaves but children and heirs.
6 And because you are children, God sent out the Spirit of his Son into your hearts, crying, “Abba, Father!”
7 So you are no longer a bondservant, but a son; and if a son, then an heir of God through Christ.
Paul warns the Galatians that their turn toward weak and miserable principles and religious calendrical observances threatens regression from sonship into slavery.
Galatians 4:8-11
8 However at that time, not knowing God, you were in bondage to those who by nature are not gods.
9 But now that you have come to know God, or rather to be known by God, why do you turn back again to the weak and miserable elemental principles, to which you desire to be in bondage all over again?
10 You observe days, months, seasons, and years.
11 I am afraid for you, that I might have wasted my labor for you.
Paul appeals to their former affection, exposes the manipulative zeal of the false teachers, and labors like a mother in childbirth until Christ is formed in them.
Galatians 4:12-20
12 I beg you, brothers, become as I am, for I also have become as you are. You did me no wrong,
13 but you know that because of weakness in the flesh I preached the Good News to you the first time.
14 That which was a temptation to you in my flesh, you didn’t despise nor reject; but you received me as an angel of God, even as Christ Jesus.
15 What was the blessing you enjoyed? For I testify to you that, if possible, you would have plucked out your eyes and given them to me.
16 So then, have I become your enemy by telling you the truth?
17 They zealously seek you in no good way. No, they desire to alienate you, that you may seek them.
18 But it is always good to be zealous in a good cause, and not only when I am present with you.
19 My little children, of whom I am again in travail until Christ is formed in you—
20 but I could wish to be present with you now, and to change my tone, for I am perplexed about you.
Paul contrasts Hagar and Sarah as an allegory of slavery and promise, linking Hagar to Sinai and the present Jerusalem, and Sarah to the Jerusalem above.
Galatians 4:21-31
21 Tell me, you that desire to be under the law, don’t you listen to the law?
22 For it is written that Abraham had two sons, one by the servant, and one by the free woman.
23 However, the son by the servant was born according to the flesh, but the son by the free woman was born through promise.
24 These things contain an allegory, for these are two covenants. One is from Mount Sinai, bearing children to bondage, which is Hagar.
25 For this Hagar is Mount Sinai in Arabia, and answers to the Jerusalem that exists now, for she is in bondage with her children.
26 But the Jerusalem that is above is free, which is the mother of us all.
27 For it is written, “Rejoice, you barren who don’t bear. Break out and shout, you who don’t travail. For the desolate have more children than her who has a husband.”
Believers are children of promise like Isaac, not children of the slave woman. The inheritance belongs to the free woman's children.
28 Now we, brothers, as Isaac was, are children of promise.
29 But as then, he who was born according to the flesh persecuted him who was born according to the Spirit, so also it is now.
30 However what does the Scripture say? “Throw out the servant and her son, for the son of the servant will not inherit with the son of the free woman.”
31 So then, brothers, we are not children of a servant, but of the free woman.