3 And first, as regards family life, it is of the highest importance that the offspring of Christian marriages should be thoroughly instructed in the precepts of religion; and that the various studies by which youth is fitted for the world should be joined with that of religion. To divorce these is to wish that youth should be neutral as regards its duties to God; a system of education in itself fallacious, and particularly fatal in tender years, for it opens the door to atheism, and closes it on religion. Christian parents must, therefore, be careful that their children receive religious instruction as soon as they are capable of understanding it; and that nothing may, in the schools they attend, blemish their faith or their morals. Both the Divine and the natural law impose this duty on them, nor can parents on any ground whatever be freed from this obligation. The Church, guardian of the integrity of the Faith-which, in virtue of its authority, deputed from God its Founder, has to call all nations to the knowledge of Christian lore, and which is consequently bound to watch keenly over the teaching and upbringing of the children placed under its authority by baptism-has always expressly condemned mixed or neutral schools; over and over again she has warned parents to be ever on their guard in this most essential point. To obey the Church in this is to obey the requirements of social utility, and to serve in the most excellent manner the common welfare. Those, indeed, whose early days were not enlightened by religious instruction, grow up without any knowledge whatever of the greatest truths, which alone can nourish in man the love of virtue, and repress in him his evil passions; such as, for instance, the ideas of God the Creator, of God the Judge and Avenger, of the rewards and punishments in another life, of the heavenly help offered to us by Jesus Christ of the conscientious and holy fulfilment of our duties. Where these are unknown, all intellectual culture will prove unhealthy; young people, unaccustomed to the fear of God, will not endure the restraint of an upright life, they will not venture even to deny anything to their passions, and will easily be seduced into troubling the State.
Source: Nobilissima Gallorum Gens (Vatican.va)