Continuing his interest in Medieval Spanish Jewry, Golden age of Jewish culture in Spain and Portugal, Netanyahu wrote a book about Isaac Abrabanel and essays on the Spanish Inquisition and the Marranos. He developed a theory according to which the Marranos converted to Christianity, not under compulsion, but out of a desire to integrate into Christian society. However, as New Christians they continued to be persecuted due to racism, and not purely for religious reasons, as was previously believed.