In verse 1, Paul is drawing on the idea that the Law with its Jewish particulars was one of the things that enslaved the Jews in a sense (along with sin and death), separating them from other peoples until the time of Christ (3:23-25), and cursing them for violation of the covenant with God. Christ, then, provided rescue from this curse and deliverance from sin and the division between Jew and Gentile. Jesus gave freedom – a new exodus, deliverance, or rescue of Israel from its exile/curse of the Law, something promised in the Old Testament to bring with it the ingathering of the nations (i.e., the Gentiles) into God’s one family. This freedom from sin, death, Jewish-Gentile division, and the Law’s curse on Israel, then, belongs to those who truly belong to God’s one promised family – as chapter 4 has it, they are the children of God’s promise to Abraham – the Sarah people, not the Hagar people still under bondage to sin, death, division, and curse.
In other words, Jesus came to fulfill God’s promise to Abraham of a single family of all nations on earth by bringing God’s salvation to the ends of the earth beginning with his exhaustion of Israel’s curse which it had acquired for covenant disobedience. This sets up verses 2-4, as this is precisely what the agitators are, in effect, denying by forcing Gentiles to become circumcised – God’s family, in their thinking, was supposed to be restricted to one nation, the Jews alone. They in effect deny the work of Christ in bringing about God’s promises. So to go back to the old use of the Law in dividing Jew from Gentile (as opposed to Jesus’ and Paul’s use) is to reject what Christ has already done, to deny his work on the cross in bringing redemption and reconciliation between the nations.
Paul’s point in verse 3, then, is that since being Jewish means, for the agitators, following all the Law’s Jewish particulars, Gentiles who obey the agitators (to become Jewish in order to become part of God’s people) are not done there – Gentiles being Jewish will have to go all the way and add to circumcision food laws, and so forth. This is not about circumcision itself per se but the motives and theology behind why these Gentiles were becoming circumcised (Paul circumcised Timothy and would not say these things in 2-4 about Timothy). Unfortunately, for centuries Gentile Christians became a version of these agitators themselves when they used this verse to deny that Jews could be Christians unless they became Gentiles first, thus again denying the work of Christ. Even today, Christians unfortunately use terms like “Jew” or “Jewish” as contraries of “Christian”, further pushing the un-Pauline view that one cannot remain a Jew and be a true Christian.
In 5 and 6, Paul turns to the true marks of God’s family. What sets them apart are not whether they are Jewish or not but whether they have faith, which is itself expressed outwardly in love, not necessarily in works of the Law (circumcision, etc.) – a love which by its very nature welcomes both Jews and Gentiles. On the basis of this life led in faith, led by the Holy Spirit (associated with freedom from sin, etc. – see, e.g., II Cor 3:17) who is the sign that the new time of faith and Israel’s rescue has come, believers may now hope for the completion of God’s work in us, fully bringing his kingdom and establishing his new people in his new creation, even among Gentiles.
In verses 7-9, Paul turns from Christ’s work to that of the agitators. These agitators are basically trying to counteract Christ’s work in bringing together a family of both Jews and Gentiles, free from enslavement. And what grieves Paul most is that it seems to be working at least somewhat! False teaching, if not checked, can easily poison the church and cause people to stumble when they are easily swayed not to attend to the truth. It takes only a few bad influences to start affecting the life of the whole church if they are allowed to continue. In verse 10, Paul is, however, confident in the Galatians’ case that they will ultimately side with him over the agitators, no matter what is going wrong at the moment, since it is ultimately the agitators themselves who are to blame for this mess.
The false teaching, hinted at in verse 11, was that Paul had kept back part of the gospel and of the full Christian life from the Galatians – the part about having to become a Jew in order to be a Christian. The position was that Paul agreed with their version of the gospel but had been too stingy and had not given the Galatians the whole thing. Summing up his self-defense so far, Paul makes it clear that he does not agree with the agitators’ version of the gospel and he certainly has not left out what they wanted to put in since it was never a part of the gospel in the first place. If he had agreed with them that Gentiles had to become Jews, he would not be persecuted by his fellow Jews (who thought he was betraying God and Moses with his message).
Paul concludes then in this section that cutting off part of your body (like in circumcision) does not matter since both Jew and Gentile are now accepted equally into a single family – why not just go all the way and be castrated rather than stop at circumcision? According to Paul, there is no significant religious difference. The irony here, of course, is that to be castrated would, by the stipulations of the Law, bar one from the religious assembly of Israel. Only the time of Israel’s rescue and the ingathering of the Gentiles, as foretold by Isaiah, would break down that barrier and allow eunuchs in on equal footing with others – precisely the work of Christ that these agitators who think they are in a privileged religious position are now denying. Paul is therefore being even cleverer here than it seems on first glance!