Then he goes on in Gal 4 to say:28 There is no longer Jew or Greek, there is no longer slave or free, there is no longer male and female; for all of you are one in Christ Jesus. 29 And if you belong to Christ, then you are Abraham’s offspring, heirs according to the promise.
After this he tells the story about Sarah and Hagar.1 My point is this: heirs, as long as they are minors, are no better than slaves, though they are the owners of all the property; 2 but they remain under guardians and trustees until the date set by the father. 3 So with us; while we were minors, we were enslaved to the elemental spirits of the world. 4 But when the fullness of time had come, God sent his Son, born of a woman, born under the law, 5 in order to redeem those who were under the law, so that we might receive adoption as children. 6 And because you are children, God has sent the Spirit of his Son into our hearts, crying, “Abba! Father!”
It's quite plain to me that here Paul is talking about the birth of Isaac. Isaac is the son born under the law, not Jesus. "When the fullness of time had come," is talking about how Sarah had to wait so long to have a child. She had to wait because she needed to show faith in God. The "we" Paul is talking about here are Jews. "We" were redeemed under the law, because "we" are children of Sarah. "You", however, are children of Hagar, so God is NOW sending the Spirit of his son into "our" hearts to redeem "you" by faith, thus you never need to become subject to the law, which is Paul's whole point in this thing. Paul is saying, now the Jews can include the Gentiles in the covenant without circumcision.
(Edited the title to say Joshua.)