Spiritual affinity: Considering spiritual kin, such as godparents, equivalent to blood relatives. Marriages between godparents and their godchildren or between godchildren and immediate members of their godparents' families were defined by the Medieval church as incestuous..

St. Boniface, acting as papal legal to the Frankish church, with the suppo rt of noble persons, took up the task of reforming marriage customs in 747. Primarily these reforms redefined the degree of relationship for determining incest. Secondarily, they introduced the concept of spiritual affinity. Roman law had never prohibited marriages between spiritual kin and even Boniface doubted how great a sin was marrying, for example, the widowed mother of your godchild.

The church's motivations for declaring that spiritual relationships had the weight of blood relationsh ips is unclear. Anthropologist Jack Good has suggested that by extending the definition of incest and therefore limiting the ability of the aristocracy to keep property in the family or build alliances with other powerful families the church was attempting to gain control of more land. --Lisa Wood