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Canon XXXI.
Canons of Local Councils Received Ecumenically (Ancyra, Gangra, Neocaesarea, Antioch, Laodicea) · The Seven Holy Ecumenical Councils
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.
LocalE 5.38.2
It is not right to give children in marriage to heretics, but they should be received if they promise to become Christians.
LocalE 5.38.5