Nicene and Post-Nicene Fathers, Ser. II, Vol. XIV:
The Canons of the Councils of Ancyra, Gangra, Neocæsarea, Antioch and Laodicea, which Canons were Accepted and Received by the Ecumenical Synods.: Canon XXXI

Canon XXXI.

It is not lawful to make marriages with all [sorts of] heretics, nor to give our sons and daughters to them; but rather to take of them, if they promise to become Christians.

Notes.

Ancient Epitome of Canon XXXI.

It is not right to give children in marriage to heretics, but they should be received if they promise to become Christians.

Van Espen.

By this canon the faithful are forbidden to contract marriage with heretics or to join their children in such; for, as both Balsamon and Zonaras remark, “they imbue them with their errors, and lead them to embrace their own perverse opinions.”