Now I say that the heir, as long as he is a child, does not differ at all from a slave, though he is master of all, 2 but is under guardians and stewards until the time appointed by the father. 3 Even so we, when we were children, were in bondage under the elements of the world. 4 But when the fullness of the time had come, God sent forth His Son, born of a woman, born under the law, 5 to redeem those who were under the law, that we might receive the adoption as sons.
6 And because you are sons, God has sent forth the Spirit of His Son into your hearts, crying out, “Abba, Father!” 7 Therefore you are no longer a slave but a son, and if a son, then an heir of God through Christ. (Galatians 4.1-7)
First, the Father is not forced to adopt. Parents adopted who they will. Adoption is a process of choice completely on the part of the parent. In the time of Paul, Roman law dictated that an adopted son received all the rights and privileges of a natural-born son. Bob Utley writes the following in his commentary on Galatians:
In Roman law, adoption was very difficult. A long, involved and expensive legal procedure, once enacted adoption afforded several special rights and privileges: (1) all debts were cancelled; (2) all criminal charges were dropped; (3) they could not be legally put to death by their new father; and (4) they could not be disinherited by their new father. In legal terms, they were a completely new person. Paul was alluding to the believers’ security in Christ by using this Roman legal procedure (cf. Rom. 8:15, 23). When a father publicly adopted a son, he officially and permanently became his heir. *
Second, the “we” in Galatians 4 I believe refers to both Jews and Gentiles (see Gal. 3.27-29) who were redeemed (bought back) by Christ. Both Jews and Gentiles used to be “under the law” (they both stood accountable before God), but Christ extracted and released them. Though they used to be slaves, now they were sons! How did they become sons? God adopted them into His family.
Third, God sends the Holy Spirit into our hearts because we are sons. The Holy Spirit in us, by Whom we cry out “Abba, Father!”, proves the fact of our adoption. See also Romans 8.9-17: the Spirit bears witness with our spirits that we are sons of God. If we have the Holy Spirit, we are sons–guaranteed! If we are sons, we have the Holy Spirit.
Fourth, if we are sons we are heirs of God, selected to receive an inheritance.
Doesn’t all that make you want to jump for absolute joy?!
* Utley, Robert James. Paul’s First Letters: Galatians and I & II Thessalonians. Vol. Volume 11. Study Guide Commentary Series. Marshall, TX: Bible Lessons International, 1997.