The critical edition by Dr. Joaquín Rodríguez Vargas of one of the major works of Persian sufism, Asrār al-Tawḥīd (The Mysteries of the Oneness of God), into Spanish, published by Mandala Ediciones in 2015, must be considered as a landmark in the field of Iranian studies in Spain. Except for the translation of some of his mystical quatrains into Spanish, the work of Shaykh Abū Saʿīd Abū l-Khayr, as well as his inmense effort towards reconciling faith and reason, had remained largely unknown in the Spanish speaking world. The translation is remarkable not only for its accuracy, but also due to the capacity of Dr Rodríguez Vargas of recreating the cultural and religious scenario of Persia in the fifth century (A H).