So there’s a period when an heir is no different from a slave. He has to do what his guardians tell him, and he can’t enjoy his inheritance. And Paul is saying that’s a picture of us. Although God chose us as his heirs before time began, we were effectively slaves until a certain date that he set.
When Paul says in v4 “when the fullness of time had come…” it seems to mean he’s talking about the Old Testament nation of Israel being enslaved until Christ came; they did their best to obey the law, and constantly made those sacrifices in the temple, but their work was never done. They were never able to finish building themselves a righteousness and then rest and enjoy their inheritance. They were enslaved to the ceremonial law. So there’s a sense in which that slavery ended with Christ – we don’t need to make sacrifices anymore because Christ’s sacrifice was sufficient, and we can rest because we have Christ’s righteousness – but I think the way Paul says “when we were children” “we… were enslaved”, “we” includes the Galatians, not just Jews like Paul himself, and the fact that he’s concerned that the Galatians might be turning back to the elementary principles they were previously enslaved to, indicates that there’s a sense in which the slavery Paul’s talking about didn’t only exist in Old Testament Israel. We were all enslaved to elementary principles before we became Christians, and we all need to beware of being drawn back into slavery. We’ll talk more about the elementary principles later on when we get to v9, but the point to begin with is you were slaves, but now you’re sons.
v4-5 – “God sent forth his son… to redeem those under the law” – Jesus went to the slave owner to buy us off of him, and the law demanded a price for us because of our sin, namely death, and Jesus paid it. And that would be great in itself, but God didn’t stop there. We’re not just set free from slavery to go and make our own way in life, working to provide for ourselves…
The point of redeeming those who were under the law was “so that we might receive adoption”. I understand adopting a child is a long and costly process. One commentator said a bit about the process he and his wife had gone through to adopt children, and apparently when people discover he has adopted children, they sometimes say things like “that’s lovely, and do you have any children of your own?” which he says is the number one thing not to say to adoptive parents, because his adopted children are his own, just as much as the biological ones, and he loves them just as much. However, this commentator admitted (understandably) that if the cost of adopting a son had been to sacrifice his firstborn, he wouldn’t have done it. But that’s exactly what God did in order to adopt us.
Jesus the Son secured our legal status as sons of God, and in v6 the Holy Spirit enables our subjective experience of sonship. Apparently the term “cry out” indicates passion, and spontaneity (a child doesn’t prepare speeches to its parents – so the Spirit in us enables us to come to God in prayer spontaneously rather than reciting mechanical phrases), and “crying out” also demonstrates a sense of God’s presence (there would be no point crying out if he wasn’t there to hear). “Abba” demonstrates assurance not just that God is there and listening, but in the same way that a child calling out for his daddy doesn’t doubt that his father loves him and will welcome him, the Spirit assures us that God loves us and welcomes our prayers.
Paul says “because you are sons, God has sent the Spirit” – As in chapter 3 v2 where he asks the Galatians “Did you receive the Spirit by works of the law or by hearing with faith?” Paul is evidently confident that they had received the Spirit. In chapter 3 he was making the point that they had received the Spirit through hearing with faith rather than by works of the law. In chapter 4 he takes a slightly different angle: the Galatians had received the Spirit because they were sons of God – because they’d been adopted – and they’d been adopted because Christ had redeemed them from under the law. Paul knew from when he’d been with them that the Galatians had received the Spirit – it had come into their hearts crying “Abba! Father!” – and that was evidence that they were God’s sons, and therefore they were no longer slaves to the law.
So if you want assurance of your adoption as one of God’s children, don’t look at how hard you’ve tried to obey the law, look for the fruit of the Spirit. The book of James is of course absolutely right that faith without works is dead, but if you’re struggling with assurance, to try to gain assurance by trying harder to do good works would be disastrous; either you’ll despair because of your persistent failures, or you’ll think you’ve succeeded – that you can be confident of your salvation because you have done enough good works. Both of those conclusions would be false. The question is not “how well have you obeyed the law?”, it’s “has God sent the Spirit of his Son into your heart crying “Abba! Father!”?”