The writings of St. Severus provides a guiding principle for Eastern and Oriental Orthodox unity. It was the same approach followed by the Church fathers remembered in Diptych. It can be achieved in Orthodoxy. Now, if a faction teach that such an unity in faith and validity of priesthood does not exist, then in the words of St. Severus "It is a great and unpardonable sin". Therefore, there cannot be a division based on stories of past errors of fathers and leaders of the Eastern and Oriental Churches. All such human errors from the past should not be taught to increase enmity. But unity in faith is most important according to Orthodox fathers.
Following from the letters of St. Severus, where the saint writes about the importance of Orthodox faith, resulting unity and validity of sacraments:
"I consider that all the bishops throughout the earth who are bound together with one another in Orthodox Faith and combined in union are one. For all follow the model of one, of the great God and our savior Jesus Christ."
"It is a great and unpardonable sin that, when the faith of the Orthodox priests is one, a man should take the holy and reverend communion from this one, and not take it from that. For it is Christ Himself and His mysterious words which are pronounced over the bread and the cup of blessing that complete the rational and bloodless sacrifice, not the priest who stands before the altar."
"withdraw therefore and dissociate yourself from such an awful and terrible judgment, and look upon all the Orthodox priests as one and as Christ Himself; and reckon the sacrifice performed by all of them to be one, and do not lay down decrees against your soul, decreeing and saying, "I will not touch any sacrifice except that which is performed by so-and-so."
"The test of true ordination is not the matter of the See, but holding the right faith in God." (Letter Concerning Ordination)