Little is known of his early life, but he was instrumental in introducing Eastern monastic practices to the West. He was raised by an affluent family, but at a relatively early age chose the monastic life. He was a disciple of St. John Chrysostom, and under his guidance and direction was elevated to the diaconate and later ordained to the priesthood while in Rome to petition on behalf of Chrystostom to Innocent I after the former’s second expulsion from Constantinople.
Cassian later founded two monastaries- one for men and one for women- and spent the remainder of his life in Marseilles. As a writer and teacher he was highly regarded, and his virtue was such that even his later critics had a high degree of respect for him.
John Cassian is notable for being the founder, in effect if not in intent, of Semi-Pelagianism. While roundly condemning the errors of Pelagius, Cassian was uncomfortable with what he perceived to be St. Augustine’s equally extreme response. Cassian hoped to forge a via media of sorts, and in doing so (probably unwittingly) stumbled in a similar error as Pelagius, albeit lesser.
In response to Augustine’s proposition that the human will, in and of itself, is unable to make a movement towards God apart from the light of grace, and in distinction from Pelagius’ understanding that the human will and faculties are essentially sound, Cassian understood that while the human will and faculties are metaphorically ‘sick,’ yet within that sickness there exists an ability (albeit one granted by God- what Cassian refers to as the “seeds of goodness…implanted by the kindness of the Creator…”) to make the first movement towards God. Cassian’s Semi-Pelagian descendents would distinguish between the ‘beginning of faith’ (initium fidei) and the ‘increase of faith;’ (augmentum fidei) the beginning of faith is proper to the human will while the increase of it (and indeed faith itself) is proper to the grace of God.
Semi-Pelagianism was ultimately condemned by the Council of Orange in A.D. 529. The 5th Canon states:
CANON 5. If anyone says that not only the increase of faith but also its beginning and the very desire for faith, by which we believe in Him who justifies the ungodly and comes to the regeneration of holy baptism — if anyone says that this belongs to us by nature and not by a gift of grace, that is, by the inspiration of the Holy Spirit amending our will and turning it from unbelief to faith and from godlessness to godliness, it is proof that he is opposed to the teaching of the Apostles, for blessed Paul says, “And I am sure that he who began a good work in you will bring it to completion at the day of Jesus Christ” (Phil. 1:6). And again, “For by grace you have been saved through faith; and this is not your own doing, it is the gift of God” (Eph. 2:8). For those who state that the faith by which we believe in God is natural make all who are separated from the Church of Christ by definition in some measure believers.
Notwithstanding Cassian’s errors, even the disciples of Augustine who were charged with refuting his teachings in this subject held him in high regard- Prosper of Aquitaine, his earliest opponent, declined to explicitly name him and continued to speak of him as a man of eminent virtue.
This selection of Cassian’s comes from his Seven Books On The Incarnation of the Lord, Against Nestorius. The larger context is the controversy over Nestorianism, which culminated in the title of Theotokos being properly applied to Mary. Cassian’s argument is that since the human nature of Jesus is united hypostatically to the divine nature of God, at any point where one might speak of Jesus one is speaking of God. To separate them is an artificial abstraction and does violence to the hypostatic union, since the human nature owes its being to the hypostatic unification with the Divine nature. Thus, since Mary is the mother of the humanity of Jesus, that humanity being at all times hypostatically united to God, she is properly named the Mother of God, via the communicatio idiomatum.
In this selection Cassian therefore argues that the divinity of God cannot be separated from himself, but in as far as the human nature of Jesus has existed the divine nature has always been united to it.
The overflowing abundance of God- his power and might and grace; even God’s very being issuing forth its eternal rays from the beginning-less past to the end-less future- belong to Jesus Christ. In every time and place, whether in the boundless expanse of the heavens or the dusty confines of the earth, even to the warmth and closeness of the womb; unto time without duration and space without measure, he is God.
Anything you could conceive about God, any word you could speak about him would find its mark in Jesus Christ, for God simply cannot stop being God. Nearer than flesh to bone, God’s God-ness is with him and in him, and this is not a fugue of speech; in every way and in every measure, he has the fullness of the Godhead.
His divinity is not a poor reflection in a dirty mirror, or a shadow of the divine that dances with its fluttering steps in the fading twilight. God cannot be less than what he is, and he cannot even be more. The same Person who is existence itself from all of eternity is the same person who walked in human flesh; the One who is high-ness itself is the same as the One who assumed our low-ness. The rags of flesh that were united to him could not cover the glory of unbridled and radiant Being.
To speak of Christ and God in the same breath is a redundancy, for he is everything that God is. In the womb of his mother all of God’s majesty and strength and being made their home, from the source of Life blossomed forth his conception. His humanity was never alone, but always was with the divine nature; how could it have been otherwise, as his existence is indebted to this union?
Because of this, whenever you read about Jesus, you read about the Son of God. It is impossible to divide the Son of God from the Godhead, for God cannot be separated from himself.