82 They, therefore, who rashly and heedlessly contract mixed marriages, from which the maternal love and providence of the Church dissuades her children for very sound reasons, fail conspicuously in this respect, sometimes with danger to their eternal salvation. This attitude of the Church to mixed marriages appears in many of her documents, all of which are summed up in the Code of Canon Law: "Everywhere and with the greatest strictness the Church forbids marriages between baptized persons, one of whom is a Catholic and the other a member of a schismatical or heretical sect; and if there is, add to this, the danger of the falling away of the Catholic party and the perversion of the children, such a marriage is forbidden also by the divine law." If the Church occasionally on account of circumstances does not refuse to grant a dispensation from these strict laws (provided that the divine law remains intact and the dangers above mentioned are provided against by suitable safeguards), it is unlikely that the Catholic party will not suffer some detriment from such a marriage.
Source: Casti Connubii (Vatican.va)