Devotion to the physical heart of Jesus can be traced back to the Middle Ages, but it was long confined to mystics. In the 16th cent. it extended to many given to the ascetic life. An elaborate theological foundation was provided by St John Eudes, but it was the visions of St Margaret Mary Alacoque in 1673–5 which gave definite shape to the object of the devotion and its practices. Its most prominent feature was reparation for the outrages committed against the Divine Love, especially in the Blessed Sacrament. It became a popular RC devotion, though the Mass and Office for the feast were not authorized until 1765; it is observed on the Friday in the week after Corpus Christi.