- His feast day is Leap Day, so he's celebrated only every 4 years.
- He is revered in the Eastern Orthodox Church and the Roman Catholic Church, but for different reasons.
- He was never officially canonized.
- He was likely a heretic.
Gregory the Great regarded him as a saint. Pope Urban V, who had been an abbot at the monastery at St. Victor, had "Saint Cassian" engraved on the silver casket that held his head. The Greek calendar of saints lists him. The modern procedure of canonization did not exist, however, and the Roman Catholic Church—although "recognizing" a feast day of 29 February—does not consider him an official saint. The Eastern Orthodox Church gave him slightly more recognition with a feast day of 23 July.
Despite these testimonies, he is considered the origin of the heretical refinement of Pelagianism called Semipelagianism. Whereas Pelagianism taught that coming to God (salvation) was an act of free will, and the orthodoxy of Augustine taught that God's grace was needed for salvation, Semipelagianism held that the initial steps toward salvation were an act of free will, but God's grace was necessary for the increase of faith that ultimately brought one to God.
It was a nice compromise, but the Second Council of Orange in 529 declared it a heresy along with Pelagianism, and reaffirmed the theology of Augustine. The 14 bishops of the Council declared that although faith was a free act, from the very beginning it was the result of God's influence. It was approved by the pope (probably Felix IV), and that was that.