Complex sentence: By then, the large majority of Jews in Spain had converted to Catholicism Conversos numbered hundreds of thousands and remained under the watchful eye of the Spanish Inquisition subject to suspicions of secret practice of Judaism, also known as "Marranism".
Split into simpler sentences: By then, the large majority of Jews in Spain had converted to Catholicism and Conversos numbered hundreds of thousands. They remained under the watchful eye of the Spanish Inquisition subject to suspicions of secret practice of Judaism by formal Catholics, also known as "Marranism".