During the 17th and 18th century, "communicatio in sacris" between Catholics and « Schismatics » of the Levant was a widespread and well-established phenomenon. The attempt carried out by the Roman Church to understand and control the ensemble of these practices gave way to a real battle of ideas, that ended up informing a much broader question: defining Oriental « Schismatics » and their treatment. The issue was: on what basis, if any, the attendance of Catholics to non-Catholic ceremonies and sacraments could be considered licit? The views and opinions analyzed in this paper take into consideration canon law and missionary manuals, reports presented by the missionaries in justification to their approach, and also the elaborate votes issued by the consultors of the Inquisition, in the effort of regulate such an intricate matter.