In a letter to his spiritual child Amphilochios of Iconium written c. 373 A.D, St. Basil the Great distinguishes three ways in which there can take place a separation of a baptised person from the communion of the Catholic Church. These three ways affecting Christian unity were said to be heresy, schism and parasynagogue, depending on whether a disagreement fell on actual faith in God, on church discipline or on ecclesiastical rulings.
(1) Heresy. From the writings of St.Basil we find that from antiquity heretics were considered to be people
“who were altogether broken off [παντελώς απερρηγμένους] and alienated [απηλλοτριωμένους] in matters relating to faith.”
Heresy is a disagreement (διαφορά), a discrepancy on vital issues of faith and culminates in the negation of the unity of God and the Church. As causes of separation (χωρισμός; αλλοτρίωσις) St. Basil mentions pride and arrogance (μεγαλοφροσύνη) originating in the human faculty of free choice (προαίρεσις).
Because it was an act of deliberate choice, heresy was not tolerated in the church. Its authors were cautioned first; then if they refused to obey, they were excommunicated from the church.
(2) Schism. The Fathers of the Church defined schism (σχίσμα) as a disagreement (διαφορά) among church members concerning ecclesiastical questions capable of mutual solution. Often (but not always) these disagreements were not of such a serious nature as to warrant a lasting feud among members of church communities.
(3) Parasynagogue. “Rival” or “counter-assemblies” were called “gatherings set up by insubordinate priests or bishops and by uninstructed people”. On this St. Basil says:
“If someone (deacon, priest or bishop) has been found in error (πταίσματι: ‘fault,’ ‘sin’)and has been asked to cease from liturgical functions but has not submitted to the canons of the Church but instead has granted to himself priestly functions and some persons abandon the Church and join him, this is parasynagogue”.
In describing the impropriety of those who originate rival assemblies St. Basil uses the term ανυπότακτος, the opposite of ευταξία, the good order and discipline of the church. Each parasynagogue or constitution of a rival assembly implies the breach of ecclesiastical unity resulting in exclusion from the Eucharistic Communion of the Church. (i.e One cuts themselves off from the communion of the Church).
Canon 5 of the Council of Nicaea (324A.D) speaks of breaches of church unity caused by unruly clergy. According to the canon the end result for the unruly clergy is ακοινώνητος γίνομαι, “to become excommunicated”. The cleric becomes excommunicated, not necessarily in the juridical term, but in the sense that unless he repents he can no longer receive Holy Communion in the Church in which alone abides the Holy Spirit.