Writing and preaching at Rome in the 390s, Jovinian argued that ascetic Christianity was based on a heretical denial of the good of God’s creation. This article points out that conformity to nature (natura) was a key element of Jovinian’s teaching as it comes down to us through Jerome. Jovinian taught that marriage was a part of human nature and demonstrably good as part of God’s creation. The word natura was also important in Jerome’s arguments against Jovinian. To refute Jovinian, Jerome offered a vision of human nature based not in empirical observation, nor in social custom, but in the actions of the incarnate Christ. In so doing, he challenged notions of human nature and social custom circulating in contemporary Rome.