The Spanish humanist Arias Montano developed several official tasks during his stay in Antwerp (1568-1575). Montano also collaborated from the beginning in the edition of the complete works of Saint Isidoro de Sevilla. In that period the project had just begun: the first step was to elaborate a catalogue of the printed and manuscript works of Isidoro kept in Spanish libraries. When that catalogue was finished, it was sent to Montano and kept in the printing office of Christophe Plantin. In this article we edit this important document and study the historical and religious context to with it belongs.