You say, ‘heretics are Christians just the same.’ Where did you take that from? Perhaps someone or other calling himself a Christian while knowing nothing of Christ, may in his extreme ignorance decide to acknowledge himself as the same kind of Christian as heretics, and fail to distinguish the holy Christian faith from those offspring of the curse, blasphemous heresies. Quite otherwise, however, do true Christians reason about this. A whole multitude of saints has received a martyr’s crown, has preferred the most cruel and prolonged tortures, prison, exile, rather than agree to take part with heretics in their blasphemous teaching.
The Ecumenical Church has always recognised heresy as a mortal sin; she has always recognised that the man infected with the terrible malady of heresy is spiritually dead, a stranger to grace and salvation, in communion with the devil and the devil’s damnation. Heresy is a sin of the mind; it is more a diabolic than a human sin. It is the devil’s offspring, his invention; it is an impiety that is near idol-worship. Every heresy contains in itself blasphemy against the Holy Spirit, whether against the dogma or the action of the Holy Spirit. (Brianchaninov, Pis’ma, no. 283; translated as “Concerning the Impossibility of Salvation for the Heterodox and Heretics”, The Orthodox Word, March-April, 1965, and Orthodox Life, January-February, 1991)