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 X

Canon X.

The members of the Church shall not indiscriminately marry their children to heretics.

Notes.

Ancient Epitome of Canon X.

Thou shalt not marry a heretic.

Fuchs.

(Bib. der Kirchenvers., pt. ii., p. 324.)

“Indiscriminately” means not that they might be given in marriage to some heretics and not to others; but that it should not be considered a matter of indifference whether they were married to heretics or orthodox.

Zonaras and Balsamon, led astray by the similar canon enacted at Chalcedon (number xiv.), suppose this restriction only to apply to the children of the clergy, but Van Espen has shewn that the rule is of general application.  He adds, however, the following:

Van Espen.

Since by the custom of the Greeks, ecclesiastics are allowed to have wives, there is no doubt that the marriage of their children with heretics would be indecent in a very special degree, although there are many things which go to shew that marriage with heretics was universally deemed a thing to be avoided by Catholics, and was rightly forbidden.