Another thing the blessed man taught and insisted upon with all was never on any occasion whatsoever to associate with heretics and, above all, never to take Holy Communion with them, 'even if', the blessed man said, 'you remain without communicating all your life, if through stress of circumstances you cannot find a community of the Catholic Church. For if, having legally married a wife in this world of the flesh, we are forbidden by God and by the laws to desert her and be united to another woman, even though we have to spend a long time separated from her in a distant country, and shall incur punishment if we violate our vows, how then shall we, who have been Joined to God through the Orthodox faith and the Catholic Church - as the apostle says: "I espoused you to one husband that I might present you as a pure virgin to Christ" [2 Cor 11:2] - how shall we escape from sharing in that punishment which in the world to come awaits heretics, if we defile the Orthodox and holy faith by adulterous communion with heretics?'
'For communion', he said, 'has been so called because he who has communion has things in common and agrees with those with whom he has communion. Therefore I implore you earnestly, children, never to go near the oratories of the heretics in order to communicate there.'
Source: Life of Saint John the Merciful