The desire of the flesh is set against the desire of the Spirit, Paul says, so that the two are in opposition (Galatians 5:17). Where they clash, the combat stymies human choice: “you may not do the things that you wish” (cf. Romans 7). By returning to the “flesh” of circumcision, the Galatians have placed themselves in that impossible position, incapable of producing the fruit they want to produce.
Why? Or, perhaps more answerable: What does flesh desire that puts it in opposition to Spirit?
Paul’s answer seems to be: Flesh desires flesh. And that in two senses.
Paul warns the Galatians that they are in danger of “biting and devouring one another” and so “consuming one another. Walking by the Spirit is walking in love (5:14), each one using his freedom to serve others (5:13). Flesh demands flesh. Flesh cannibalizes.
More specifically, the Judaizers – children of the flesh, children of Hagar (4:25, 29) – want the flesh of the Galatians, literally the foreskin of their flesh (6:12-13), so they can “boast” in the flesh (the cut flesh, the absent flesh) of the Galatians.
Now, it seems that Paul sees something in the institutions of the Law that feeds the flesh’s desire for flesh. He not only sets flesh against Spirit, but sets the leading of the Spirit against the domination of the law (“under” law, 5:18). As a “fleshly” set of institutions and practices, Torah somehow seems to encourage the flesh’s appetite for flesh, and resulting honor-boasting.
If there is to be harmony rather than combat, service rather than consumption, there has to be a liberation from the law, so that those who follow the Spirit can fulfill the law (5:14), the law of Christ (6:2).