2 In order, however, that amongst men of every nation and every age the desired fruits may be obtained from this renewal of matrimony, it is necessary, first of all, that men's minds be illuminated with the true doctrine of Christ regarding it; and secondly, that Christian spouses, the weakness of their wills strengthened by the internal grace of God, shape all their ways of thinking and of acting in conformity with that pure law of Christ so as to obtain true peace and happiness for themselves and for their families.
Source: Casti Connubii (Vatican.va)