Jeff VanVonderen in Families Where Grace In In Place says,
Paul was angry at the Galatians. His letter to them is a scathing confrontation of the fact that they had been "walking by the flesh" instead of the Spirit. But the "flesh" for the Galatians was not pornography, drunkenness, or thievery. The Galatians had begun to measure their acceptance spiritually by whether or not they performed certain religious behaviors. They let religious performance direct the way they acted, instead of allowing that Spirit to do so. Paul calls this "walking in the flesh."
I saw this statement quoted and looked it up to see if he actually said this. He actually did.
What’s interesting is what the Apostle Paul actually said to the Galatians:
Now the deeds of the flesh are evident, which are: immorality, impurity, sensuality, 20 idolatry, sorcery, enmities, strife, jealousy, outbursts of anger, disputes, dissensions, factions, 21 envying, drunkenness, carousing, and things like these, of which I forewarn you, just as I have forewarned you,...