Second Council of Nicaea, Canon VIII

That Hebrews ought not to be received unless they have been converted in sincerity of heart.

Since certain, erring in the superstitions of the Hebrews, have thought to mock at Christ our God, and feigning to be converted to the religion of Christ do deny him, and in private and secretly keep the Sabbath and observe other Jewish customs, we decree that such persons be not received to communion, nor to prayers, nor into the Church; but let them be openly Hebrews according to their religion, and let them not bring their children to baptism, nor purchase or possess a slave.  But if any of them, out of a sincere heart and in faith, is converted and makes profession with his whole heart, setting at naught their customs and observances, and so that others may be convinced and converted, such an one is to be received and baptized, and his children likewise; and let them be taught to take care to hold aloof from the ordinances of the Hebrews.  But if they will not do this, let them in no wise be received.

Translation: Henry R. Percival, The Seven Ecumenical Councils, NPNF II.14 (1900), via CCEL.