The article analyses how the decrees of the Council of Trent regarding marriage were used by the Church of Rome as a tool to contrast mixed marriages in Early Modern Europe. It investigates how these decrees were evaded by local churches in order to administer a practice of confessional coexistence impossible to eradicate, and how they were manipulated by actors – even Protestants – to put an end to undesirable unions. It also presents the interpretation that the Church of Rome made of the Tametsi to resolve the age-old issue of mixed marriages in the Low Countries, issuing the Benedictine Declaration, later applied to other contexts with a strong Protestant presence – above all out-side Europe. Although the Council of Trent claimed to have fixed a homogeneous and flawless nuptial ritual, the various local practices did not always adapt to it. Indeed, they bypassed it; sometimes refused it. This led parish priests and missionaries to turn to Rome for the resolution of concrete cases. The decisions taken for individual cases became a normative reference point. It was produced by the continuous interaction and negotiation with local churches and went on in fact to profoundly influence the sacramental rituality of marriages, which Tametsi had claimed were fixed and immutable.