Let no Orthodox man be allowed to contract a marriage with a heretical woman, nor moreover let any Orthodox woman be married to a heretical man. But if it should be discovered that any such thing is done by any one of the Christians, no matter who, let the marriage be deemed void, and let the lawless marriage tie be dissolved. For it is not right to mix things immiscible, nor to let a wolf get tangled up with a sheep, and the lot of sinners get tangled up with the portion of Christ. If, therefore, anyone violates the rules we have made let him be excommunicated. But in case persons who happen to be still in the state of unbelief and to be not yet admitted to the fold of the Orthodox have joined themselves to each other by lawful marriage, then and in that event, the one of them having chosen the good start by running to the light of truth, while the other, on the contrary, has been held down by the bond of delusion for having failed to welcome the choice of gazing at the divine rays (whether it be that an unbelieving woman has looked with favor upon a man who is a believer, or vice versa an unbelieving man upon a woman who is a believer), let them not be separated, in accordance with the divine Apostle: “For the unbelieving husband is sanctified by the wife, and the unbeelieving wife by the husband” (1 Cor. 7:14).
Canon 72, Council of Trullo (Sixth Ecumenical Council)
There is a sad state of much disobedience in this matter and allowance of mixed “marriages” with those outside of the Church. Disobedience to Christ is bad enough in this matter but let us examine what is happening here and hopefully those of a right mind will be repulsed by the idea of such a marriage and never trouble the clergy to permit such a thing.
Why is my language so strong? Why have a put quotes around the word marriage? It is because once the Mystery of marriage is better understood, it will be realised that there is true marriage between someone in the Church and one outside, not in the Mystery of marriage. There is a union in the flesh but this only serves to heighten the problem.
Marriage is the Mystery of the union of Christ and His Church. The man is the icon of Christ and the woman is the icon of the Church. Their union as husband and wife is the union of Christ and the Church. They become one flesh in Christ. Each family is a small Church in Christ and thus marriage finds its place in Christ. As with all aspects of Christian life our life is in Christ and everything finds its place in Him. If it cannot then it is to be rejected from our lives because what is not in Christ is dead.
Only a baptised man in the Church can be Christ because only such a man can truly have Christ and be in Christ. Only a baptised woman in the Church can be the Church, the body of Christ because only in the Church does one become united to His body and partake of it. So the Mystery of marriage can only be fulfilled between those in the Church. To marry someone outside the Church in Christ is impossible because those outside the Church cannot fulfil their place in the Mystery of marriage and the marriage is not a true marriage in Christ but something outside him and of the flesh. (It should nevertheless be respected as marriage in that there is still a union on flesh and to avoid an excuse for adultery. Also, in respect of secular law that recognises the union.)
This is not to mention that when a believer lives as one flesh with an unbeliever then they are uniting the flesh of Christ with one unbaptised and this confers a similar burden on their soul as if the unbeliever were to dare to receive communion. Would you dare to share holy communion with heretics? Then why share your body, or rather Christ’s body with them in marriage? Such a thing should not occur.
An exception to this is when one partner comes into the saving body of the Church and the other remains outside or if one’s partner should decide later to leave the Church. In this case the faithful partner sanctifies his/her partner for the sake of the children. They should not depart because this would case many other problems but if the unbeliever decides to leave of his/her own will then so he/she should be let free. There is no true “marriage” in Christ and so no breach of the Mystery. It is a state of economy but not the fulness of the Mystery in Christ.