Paul's use of in Rom 12.1 has long fascinated and puzzled interpreters. This study proposes a new explanation of Paul's reason language in Rom 12.1 based on a detailed investigation of ancient philosophical texts on the role of human beings in the cosmos, in which reason language and the idea of a vocation of human beings are closely connected. It argues that Paul here appeals to the idea of a human vocation in order to claim that Christ-followers are able to fulfil their human vocation by living in such a way that their lives produce signs of the new creation inaugurated in Christ. This case is made by establishing the central role of reason in ancient discourse on what it means to be human more broadly, and in particular in Epictetus, who provides the clearest parallel for Romans. These contextualisations allow for a fresh reading of Paul's argument in Romans, where the relevance of these traditions is shown, not least for how Rom 12.1-2 frames Rom 12-15. The study thus contributes to the recent scholarly trend of exploring Paul in ancient philosophical contexts and advances the discussion on the integration of Paul's "theology" and "ethics" within an ancient cultural encyclopedia.