Casti Connubii §83

⁸³ Whence it comes about not unfrequently, as experience shows, that deplorable defections from religion occur among the offspring, or at least a headlong descent into that religious indifference which is closely allied to impiety. There is this also to be considered that in these mixed marriages it becomes much more difficult to imitate by a lively conformity of spirit the mystery of which We have spoken, namely that close union between Christ and His Church.