Can nationalism be heresy? Augustine's social thought resisted both the religious nationalism of those who saw Rome's conquest of neighboring nations as God's redemptive work, and the religious isolationism of the Donatist sect that rejected all Christian involvement in Rome's public life. Augustine diagnosed both the nationalist and the isolationist temptation, not merely as unethical and imprudent, but as unorthodox and heretical-trusting in salvation by human works. His arguments against nationalism and isolationism mirror those he was using at the same time against the Pelagians; for Augustine, religious nationalism was missiological Pelagianism, while religious isolationism was ecclesiological Pelagianism. His writings against these two errors establish that fidelity to the gospel requires us to distinguish, but not separate, religious institutions and identities from political institutions and identities, creating a complex and ambiguous shared space within which the city of God and the worldly city engage in spiritual struggle. This fresh and innovative look at Augustine's social thought, from an interdisciplinary perspective that draws on theology and social science, speaks directly to questions that are urgent in every age of the church, including our own.