Against Schism From Orthodoxy
I agree with Craig Truglia, who wrote: "St optatus wrote to the Donatists, who had rigorist credentials, how do they disagree with the whole world? And so, while there are heretics within the Church, where is the evidence that all the synods are teaching an explicit heresy...not a bad bishop here or there, but a synodical meeting espousing heresy and all the synods doing this? I think we are no where close to how bad things were in the 7th century so I'm not seeing the imminent catastrophe. This is without getting into the fringe and weird things within the entire genuine Ortho movement, which does not mean every single person or bishop is bad but the whole movement has strange things as well. I don't see the appeal of jumping off a supposedly shrinking ship to a capsized one. That's how I personally feel."
This is not an exhaustive treatment.
In the Bible unity is very important (Eph. 4:12-13). The Christian is to avoid divisions (Rom. 16:17-18). Those who promote "strife...dissension, party split...shall not inherit the Kingdom of God." (Gal. 5:20-21). St. Jude warns that those who "set up divisions" are worldly people, devoid of the Spirit." (Jude 19).
Many scholars believe the Epistle of Pseudo-Barnabas (c. 70-130) to be the earliest of all post-New Testament extrabiblical sources. This epistle is addressed to "sons and daughters" in the faith. They are urged to avoid any "schism" and make peace between contending factions.
Saint Augustine: "To Optatus, as with Cyprian, schism is tantamount to apostasy." (Kelly, Early Christian Doctrines, 411-412).
Saint Ambrose and Saint Augustine regards heretics and schismatics as blasphemers against the Holy Spirit insofar as they deny the Spirit and Truth that is in the True Church.
Saint Ignatius of Antioch said: “Make no mistake, brethren, no one who follows another into schism will inherit the Kingdom of God, no one who follows heretical doctrines is on the side of the Passion.”
Saint Ignatius teaches that there is no sin worse than the sin of division in the Church, and that not even the blood of martyrdom will wash it away.
Saint John Chrysostom writes: "I do say and affirm that schism is just as terrible an evil as heresy."
Saint John Chrysostom said: "I say this for those who indiscriminately go to all churches--both to ours and to those of the schismatics. If they teach differently than we do--then for that very reason, of course, one ought not to go to them. But if they teach the very same as we do--then all the more cause why one ought not to go to them, for here is the sin of lust for authority."
Saint Cyprian of Carthage writes: If such men were even slain in confession of the Christian name, not even by their blood is this stain washed away...He cannot be a martyr, who is not in the Church.
In the life of Saint Anthony (written by Saint Athanasius), it says in chapter 91: Have no fellowship [communion] with the schismatics, nor any dealings at all with the heretical Arians.
St. Basil the Great instructs: Because their separation [is] initiated through schisms ... those who [have] separated themselves from the Church no longer [have] in themselves the grace of the Holy Spirit.... Nor could they any longer confer on others that grace of the Holy Spirit from which they themselves had fallen away.
Saint Optatus, Bishop of Milevis, wrote in the 4th century:
"Now by the commandments of God, three things are, amongst others, forbidden by him. Thou shalt not kill; thou shalt not worship strange gods, and summing up the commandments, thou shalt not commit schism."
Canon 33 of the Local Synod of Laodicea states: "That one must not pray together with heretics or schismatics”
Saint Basil stated: such a schism, is already in all things like under heresy, and one must treat such schismatics as one would heretics, not permitting any communion with them.
Canon VI of the 7th Ecumenical Council states in relevant part: "We call those heretics...who, though pretending to confess the sound faith, have schismatically separated and have gathered congregations in opposition to our canonical bishops."
The incorrupt Saint Metropolitan Philaret taught that schismatics do not have the grace of the sacraments. See his letter to Abbess Magdalena, written in 1979.