St. Cassian lived in the 4th century, hardly enough time for the complete linguistic aparthied, and certainly hardly enought time to develop "unrecognizable" praxes on both sides. In St. Cassian's time the filioque was not an isse even in Spain (it was to wait another 200 years for that to be initiated).
Yet, the West was already divided on the issue of "original sin" which the Easter managed to ignore until much later, just as the West managed to ignore the Capadoccian Fathers' clear cut assertion that we can not know God's essence, but only His uncreated enegries.
Thus, lots of theological discrapancies existed in the so-called "undivided" Church of the firts millennium, yet they intercommuned freely, consider the unresolved issues something that had to be worked out, but at the same time no one doubted that it was one and the same Church in its core.
- Death did not come to Adam from a physical necessity, but through sin.
- New-born children must be baptized on account of original sin.
- Justifying grace not only avails for the forgiveness of past sins, but also gives assistance for the avoidance of future sins.
- The grace of Christ not only discloses the knowledge of God's commandments, but also imparts strength to will and execute them.
- Without God's grace it is not merely more difficult, but absolutely impossible to perform good works.
- Not out of humility, but in truth must we confess ourselves to be sinners.
- The saints refer the petition of the Our Father, "Forgive us our trespasses", not only to others, but also to themselves.
- The saints pronounce the same supplication not from mere humility, but from truthfulness.
- Some codices containing a ninth canon (Denzinger, loc. cit., note 3): Children dying without baptism do not go to a "middle place" (medius locus), since the non reception of baptism excludes both from the "kingdom of heaven" and from "eternal life".
How about the mentioning of Original sin in (2)? Do the Orthodox view the Carthage Council of 418 as binding?