Nicene and Post-Nicene Fathers, Ser. II, Vol. XIV:
The Canons of the Council in Trullo; Often Called The Quinisext Council.: Canon LXXII

Canon LXXII.

An orthodox man is not permitted to marry an heretical woman, nor an orthodox woman to be joined to an heretical man.  But if anything of this kind appear to have been done by any [we require them] to consider the marriage null, and that the marriage be dissolved.  For it is not fitting to mingle together what should not be mingled, nor is it right that the sheep be joined with the wolf, nor the lot of sinners with the portion of Christ.  But if any one shall transgress the things which we have decreed let him be cut off.  But if any who up to this time are unbelievers and are not yet numbered in the flock of the orthodox have contracted lawful marriage between themselves, and if then, one choosing the right and coming to the light of truth and the other remaining still detained by the bond of error and not willing to behold with steady eye the divine rays, the unbelieving woman is pleased to cohabit with the believing man, or the unbelieving man with the believing woman, let them not be separated, according to the divine Apostle, “for the unbelieving husband is sanctified by the wife, and the unbelieving wife by her husband.”

Notes.

Ancient Epitome of Canon LXXII.

A marriage contracted with heretics is void.  But if they have made the contract before [conversion] let them remain [united] if they so desire.

Perhaps none of the canons of this synod present greater and more insolvable difficulties than the present.  It has been for long centuries the tradition of the Church that the marriage of a baptized Christian with an unbaptized person is null, but this canon seems to say that the same is the case if the one party be a heretic even though baptized.  If this is what the canon means it elevates heresy into an impedimentum dirimens.  Such is not and never has been the law of the West, and such is not to-day the practice of the Eastern church, which allows the marriage of its people with Lutherans and with Roman Catholics and never questions the validity of their marriages.  Van Espen thinks “the Greek commentators seem” to think that the heretics referred to are unbaptized; I do not know exactly why he thinks so.