43 Care also must be taken that they do not easily enter into marriage with those who are not Catholics; for, when minds do not agree as to the observances of religion, it is scarcely possible to hope for agreement in other things. Other reasons also proving that persons should turn with dread from such marriages are chiefly these: that they give occasion to forbidden association and communion in religious matters; endanger the faith of the Catholic partner; are a hindrance to the proper education of the children; and often lead to a mixing up of truth and falsehood, and to the belief that all religions are equally good.
Source: Arcanum Divinae (Vatican.va)