Antony Mecherry S.J. brings to the fore a recently identified 16th-century treatise on 'Nestorianism', written by Francisco Ros S.J. (1559-1624), a Catalonian from the Jesuit province of Aragón. He successfully promoted the mission praxis of accommodatio primarily among the Saint Thomas Christians of early modern Malabar in South India. This newly discovered first treatise composed by Ros, a Latin missionary, represents the initial phase of his mission as a polemicist in the making, who read the Syriac sources of the Church of the East found in Malabar through a Catholic theological lens. In addition to exploring the underlying conflicts which emerged out of an unprecedented encounter of apparently unlike theological and liturgical identities in the same mission field of early modern India, this book provides the readers with a historiographical critique against the backdrop of which the author presents his analysis of the Rosian treatise.