This essay is a social-scientific study of Paul's deployment of holiness language in 1 Corinthians. Specifically, an interpretation of holiness is offered to explain Paul's argument in 1 Cor 7.12–16 in favour of non-separation in the case of a believer married to a non-believer. For Paul, holiness involves participation in the oneness of God interpreted christologically. This participation is embodied in the holiness-as-oneness of the church. In relations between believers and unbelievers, purity rules to do with sex and marriage carry a significant symbolic burden. In some cases, clear lines of demarcation are drawn. Other cases constitute grey areas; and the suggestion here is that ‘mixed marriages’ are one such. For Paul, holiness is a matter of neither genealogical nor cultic purity. Rather, it has a boundary-transcending quality. In the case of a mixed marriage, the unbelieving partner, together with the children, is sanctified by remaining in oneness with the believing partner. Paul's concern for the oneness of the church spills over into a concern for the oneness of the household.