The Catholic Encyclopedia

The Anima Christi was and is still generally believed to have been composed by St. Ignatius Loyola, as he puts it at the beginning of his "Spiritual Exercises" and often refers to it. This is a mistake, as has been pointed out by many writers, since the prayer has been found in a number of prayer books printed during the youth of the saint and is in manuscripts which were written a hundred years before his birth (1491).