26 With regard to those marriages in which one or the other party does not accept the Catholic teaching or has not been baptized, We are certain that you observe exactly the prescriptions of the Code of Canon Law. Such marriages, in fact, as is clear to you from wide experience, are rarely happy and usually occasion grave loss to the Catholic Church. A very efficacious means for driving out such grave evils is that individual Catholics receive a thorough training in the Divine truths and that the people be shown clearly the road which leads to salvation.
Source: Sertum Laetitiae (Vatican.va)