Scholars have traditionally assumed that Christian Hermetism emerged when renaissance figures like Pico della Mirandola and Giordano Bruno harmonized the wisdom of the legendary sage Hermes Trismegistus with Christian doctrine and classical philosophy. However, I suggest that an earlier effort was made by the Castilian king Alfonso X, known as “the Learned,” in the thirteenth century. From the start of his rule, Alfonso supported the translation and organization of classical, Arabic, and Jewish traditions in a secular manner, with a focus on astrology and magic. At the end of his life, Alfonso faced accusations of heresy from the Church and his rebellious son. These conflicts forced him to defend himself in a complex and unfinished work: the Setenario.