John Cassian was a zealous monk whose theology (unfortunately, one might say) has been massively influential on the church’s understanding of the whole of the gospel since the fifth century. His particular theology (commonly known as semi-Pelagianism), which was developed largely in response to Augustine’s doctrines of predestination, grace, and free will, has been adopted by many Christians—academics, clergy and lay people alike—throughout the centuries.Two major influences were at work in Cassian’s life and teachings. First, Greek neo-platonic philosophical theology shaped his understanding of anthropology in a way that prevented him from being able to engage Augustine on the level that he should have. And second, his intense devotion to the ascetic chastity of the monastery created a platform upon which his theology could develop, yet in a way that was almost entirely sub-biblical. The result of Cassian’s theological contributions to the church has been the obscuring of the God of the Bible in the vision of His people.
Cassian and His Work
Cassianus was born (probably in Provence) around 360 A.D., and most likely assumed the name “Iohannes” (John) at his baptism or admittance to the monastic life.  He died in Massilia of Gaul (present-day Marseilles, France), where he had spent his most productive years as a monk, in 435.  His birthplace is uncertain, and little is known about his parents, education, or childhood, primarily because of his own silence regarding these in his writings.
Augustine’s influence and authority had been growing since the official defeat of Pelagianism, which was condemned in 418 at the 16th Council of Carthage.  The ‘initial spark’ was provided for the Cassian controversy when one of Augustine’s letters, concerning predestination and prevenient (and therefore irresistible) grace, came into the possession of monks at Adrumetum. Dispute arose among them over these doctrines, and they eventually sent a dispatch to Hippo to ask Augustine about the fuller meaning of his writings.  So Augustine wrote De Gratia et Libero Arbitrii (On Grace and Free Will) and De Correptione et Gratia (On Rebuke and Grace) in 426, with the hope of clarifying the matter. 
Cassian’s ‘Solution’ Examined
Most of Cassian’s relevant arguments are laid out in the 13th book of his Conferences, which is a record of a conversation with Abbot Chaeremon entitled “On the Protection of God,” though he does touch upon the same doctrines, to lesser extents, in several other places. Methodologically, it must be said—to his commendation—that he uses Scripture with great frequency.For Cassian, and others in opposition to strong Augustinianism, it seems there were two factors of primary concern in the debate. First, being a monk whose daily life consisted of disciplined asceticism for the sake of chastity (moral purity), Cassian feared that Augustine’s doctrines would give an overwhelming sense of powerlessness and despondence in such pursuits. This, in turn, might lead to ethical irresponsibility (the lack of the feeling of accountability). 
It is of utmost importance to note that Cassian “positions
his analyses of grace and free will within his discussions of chastity.”
The crucial issue for him was the empowerment
for the pursuit of holiness. So great
was his concern for chastity, in fact, that earlier in his life Cassian gave
up the solitary life of an Anchorite monk in Egypt for that of a Coenobite in
community with other monks, “in order that he might have the opportunity of
practicing the virtues of obedience and subjection, which seemed out of the
reach of the solitary.”
unlike Pelagius, however, Cassian insisted that divine grace was absolutely
necessary for spiritual progress. “How
foolish and wicked then it is to attribute any good action to our own diligence
and not to God’s grace and assistance, is clearly shown by the Lord’s saying,
which lays down that no one can show forth the fruits of the Spirit without
His inspiration and co-operation.”
Instead, he sought some middle ground of cooperation
between man’s willful initiative and God’s enabling grace (libero arbitrio
Cassian’s ‘Solution’ Refuted
Second, and closely related to the first, is the weak view of sin and grace in Cassian. Sin for him seems to be only a violation of command and conscience. For Augustine, and in Scripture, the essence of sin is more than this—it is a rejection of the supremacy of the glory of God for delight in things of infinitely less worth… and therefore much more dishonoring to God. Accordingly, Cassian’s view of grace is more Pelagian than Augustinian. For him grace is merely an agent of enabling unto holiness (seeing Christ more as an instructor than a savior). He would likely have a low view of the substitutionary atonement of Christ. Third, there seems to be in Cassian the attempt to maintain some level of autonomy from God in the process of salvation. This is perhaps the point where Augustine sees Cassianism “as necessarily implying the basal idea of Pelagianism,”  thereby referring to it as “semi-Pelagianism.” Indeed, prior to regeneration we are all born Pelagians,  at our religious “best” hoping to commend ourselves to God by some means other than Christ and the total reliance upon the sovereign grace of God.
The Official Outcome
Prosper of Aquitaine, lay friend of Augustine, took up the defense of monergism against Cassian’s synergism from the beginning. He had alerted Augustine to the trouble in Massilia, motivating the bishop to write the two last works of his life against semi-Pelagianism (De Praedestinatione Sanctorum and De Dono Perseverantiae). And shortly after Augustine’s death he, with his (otherwise unknown) companion, Hilary, petitioned Pope Celestine to condemn Cassian’s teachings. However, they encountered difficulty in
…trying to secure a condemnation of someone (Cassian) who had just written the anti-Nestorian treatise The Incarnation of the Lord at the behest of Leo, archdeacon of Rome. Cassian had, surely not coincidentally, linked Nestorius to Pelagius and attached both vigorously. His stock was high in Rome. 
The Abiding Influence
Tragically for the church, Augustinianism was being softened by the bishop’s successors before semi-Pelagianism was even officially condemned.  It was not held to strongly enough for the fundamental tenets of Augustine’s theology to take deeper root in the church. The resulting influence of Cassian has been widespread and long lasting. For, while it is true that the Council of Orange was a triumph over the “semi-Pelagian denial of the necessity of prevenient grace for salvation,” Robert L. Reymond observes,
…that same council betrayed the church into the semi-semi-Pelagian denial of the irresistibility of that prevenient grace by human free will, which theological vision, in spite of the recurring protests through the centuries of such men as Gottschalk, Bradwardine, Wycliff, and Hus, eventually an Aquinas was to systematize and the Council of Trent (1545) was to declare the official position of those churches in communion with Counter-Reformation Rome. 
 Edgar C. S. Gibson, preface to The Works of John Cassian, by John Cassian, trans. Edgar C. S. Gibson, in The Nicene and Post-Nicene Fathers, 2nd ser., vol. 11, ed. Philip Schaff, accessed through The Master Christian Library, ver. 8 (Rio, WI: AGES Software, Inc., 2000), 375.
 The New Schaff-Herzog Encyclopedia of Religious Knowledge (Grand Rapids: Baker Book House, 1954), s.v. “Cassianus Johannus,” http://www.ccel.org/php/disp.php?authorID=schaff&bookID=encyc02&page=435&view=
 Columba Stewart, Cassian the Monk, Oxford Studies in Historical Theology (Oxford:
Oxford University Press, 1998), 5.
 Gibson, 383.
 Stewart, 5.
 The Catholic Encyclopedia, 1908 ed., s.v. “John Cassian,” http://www.newadvent.org/cathen/03404a.htm
 Gibson, 383.
 John Cassian, On the Incarnation against Nestorius, in The Works of John Cassian, trans. with preface Edgar C. S. Gibson, in The Nicene and Post-Nicene Fathers, 2nd ser., vol. 11, ed. Philip Schaff, accessed through The Master Christian Library, ver. 8 (Rio, WI: AGES Software, Inc., 2000), 1:3.
 Gibson., 387.
 Cassian., Against Nestorius, 1:4.
 Robert L. Reymond, A New Systematic Theology of the Christian Faith, 2d ed. rev. & updated (Nashville: Thomas Nelson, 1998), 468-9.
 B. B. Warfield, introduction to Saint Augustine’s Anti-Pelagian Works, by St. Augustine, trans. Peter Holmes & Robert Ernest Wallis, rev. B. B. Warfield, in The Nicene and Post-Nicene Fathers, 1st ser., vol. 5, ed. Philip Schaff, accessed through The Master Christian Library, ver. 8 (Rio, WI: AGES Software, Inc., 2000), 89-90.
 Gibson, 388.
 Ibid., 389.
 Warfield, 97-8.
 St. Augustine, The Predestination of the Saints, in Saint Augustine’s Anti-Pelagian Works, by St. Augustine, trans. Peter Holmes & Robert Ernest Wallis, rev. with intro. B. B. Warfield, in The Nicene and Post-Nicene Fathers, 1st ser., vol. 5, ed. Philip Schaff, accessed through The Master Christian Library, ver. 8 (Rio, WI: AGES Software, Inc., 2000), ch. 13, emphasis mine.
 Ibid., ch. 14. See Rom. 9:22-23. For an excellent treatment of the righteousness of God in His sovereign election based on this passage, see John Piper, The Justification of God: An Exegetical & Theological Study of Romans 9:1-23, 2nd ed. (Grand Rapids: Baker Books, 1993), especially pp. 183-216.
 Gibson, 389
 Earle E. Cairns, Christianity through the Centuries, 3rd ed. rev. & expanded (Grand Rapids: Zondervan, 1996), 132.
 Stewart, 76.
 Gibson, 378.
 Cassian, The Conferences of John Cassian, in The Works of John Cassian, 3:16.
 Stewart, 19.
 R. C. Sproul, Willing to Believe (Grand Rapids: Baker Books, 1997), 73.
 Cassian, Conferences, 13:8.
 Sproul, 70.
 Cassian, Conferences, 13:7.
 Stewart, 78.
 Warfield, 93.
 Reymond, 469.
 Stewart, 20-21.
 Gibson, 390-391.
 Sproul, 75.
 J. N. D. Kelly, Early Christian Doctrines, rev. ed. (San Francisco: Harper Collins, 1978), 371-2.
 Sproul, 75.
 Reymond, 469