It is very important that we not lose sight of Paul's point in chapter 3 that the purpose of the Law was not to "save" Israel, but rather to point it to Jesus, the Messiah. Thus, Christians need to be careful that they use the Law and the Gospel properly. It has been said that the Law thunders from Mt. Sinai and uncovers sin to bind all under guilt. But the Gospel speaks softly from Golgatha and offers forgiveness of sin and sonship with Jesus. The key is to understand that the Law teaches us why we need the Gospel. So Christians can use the law to convince people that they are sinners, but should never use the law to "convince" other believers in the Gospel of Jesus, that they can't be saved because of their lack of performance.
In Galatians chapter 4, Paul begins to more fully explain the truth that in Jesus we have become "sons of God." He does this by pressing home more fully the analogy of a child-heir. The first point he makes is that as long as the heir is a child it is functionally a slave. There is no current difference even though things will change in the future. The Father puts the child in the hand of guardians and tutors in order to prepare it for a life of working with the Father and eventually taking over for him. The second point is that this situation always has an appointed end. In fact in vs. 4 it is phrased this way, "when the fulness of time had come." This "fulness" can be pictured like a ship that is being loaded with its shipment. When the bill of shipping has been completely loaded onto the ship then it is full. Paul compares these two points with Israel.
Israel was in such a relationship. God made Israel his child and placed it under the supervision and training of the Law. But this relationship had an appointed end. That appointed time is both a quantity and a quality of things to occur. When the time arrived that God had appointed then the Messiah, Jesus, would come forth and lead Israel into adult sonship. Paul makes sure to emphasize that Jesus was in every way an Israelite, born of a woman, under the law. Thus the Messiah identified with Israel and suffered with them in their trials, in bondage to the elements of this world.
Paul inserts the idea of redemption in verse 5. This goes beyond the analogy. Israel was not just in need of training. But in a way similar to how Gomer was so adulterous that she sold herself into slavery, so Israel as God's child was a slave to sin and needed to be redeemed or bought back. In this sense the prodigal son cannot get up and come home because he is in jail and his fine must be paid before he can go home. Jesus came to pay their fines so that they could experience adult sonship in God's family.
Paul's use of the word adoption is somewhat confusing at first in light of the analogy. But as I have said Paul has begun to go beyond the analogy and somewhat leave it in the dust as he explains the greater truth of what Jesus has done for Israel. Adoption makes sense when we look at the Gentiles. They did not know God and were never part of his family. Thus they are being adopted into his family. But how can we say that Israel is adopted? Perhaps it would be best to ask ourselves how the Gentiles were so estranged from God. No matter what if you follow any nation's history backwards you will eventually get to Noah, Ham, Shem, and Japheth. That is somewhere back there they all "knew" the truth. There had to be a process of rejecting the truth that became so entrenched and so far back into the past that they were completely spiritually lost. I believe that Paul is saying a similar process had gone on in Israel in spite of God's continual faithfulness to speak to Israel through the prophets. Though God had treated Israel as a son, because of its sin it was a slave in prison completely estranged from God and in need of redemption and adoption.
Paul has made it very clear that the adoption of sons is connected to putting our faith in Jesus Christ in Galatians 3:26. Thus putting our faith in Jesus Christ becomes that final test to entering into adult-heir status. Those who believe in Jesus are given the indwelling presence of the Holy Spirit. This is a picture of the new birth that Jesus speaks of in John 3. Being born from above, born again, or born of the Spirit is the point at which our spirit-man becomes alive to God. Before this we may be religious, pray, and sing worship songs, but our spirit is not alive to God. Our flesh is running the show satisfying desires that are not as altruistic as they appear on the surface. But when the Spirit of God dwells in our hearts, He enables our hearts to call God, Abba Father. A term that a child would use of a Father. It means a close intimate relationship with God our Father. This means we are no longer slaves under the Law but sons who are heirs and co-workers with God in his business of saving men through Christ.
This does bring up a point of contention. Some Christians seem to cop an attitude that says something to the effect that since I'm an adult son and not under the Law then I can do anything I want. God's grace covers it all. I know this isn't spoken of in these verses, but give a second to tie this in.
The question really is this, Does the Law have anything to say to us any more? Once the child becomes an adult his relationship with the tutor changes, but it doesn't necessarily sever, does it? In fact governments have long understood that laws must be passed in order to have a society, much less a civil one. So the Law can give us understanding into man's sinful nature as we craft civil laws. Also the same lessons that the Law taught us we can teach to others who have not been under the tutor. Thus the Gentiles are made aware of their sin and coming judgment. The apostles pleaded with Gentiles to save themselves from their wicked and perverse generation in order to avert the judgment of God. The law can also personally give us wisdom and insight on those things that tend to bondage in life and seek to avoid them. Thus Christians have not graduated into a life of lawlessness, but rather are now above the law. It has been said that we walk upon the law instead of being tread down by it, which is a fair point. However, even in that we must be careful that we are not trying to re-establish the performance of works in our approach to God. We now have an intimate, adult, relationship with God by which we need not fear and yet, we still desire to be like him. May the Spirit of God continue to cry out through us Abba Father, as we give ourselves to his business of saving the lost.