Usually, I see this term in regards to marriage, and people not being unequally yoked.
Ideally, a couple should be of the same faith. Obviously, people will marry outside of their faith, or even within their faith, but to (for Christianity), a different denomination. A different cult. When this happens with Catholics, the Catholic spouse has the obligation to raise their children Catholic.
I can imagine how confusing this must be for the children. They are being taught one thing, but one parent, even in the most amicable of households, doesn't follow that tradition. It makes sense that the better way, the preferred way, would be for the parents to be united. Otherwise, one parent must compromise their faith, even in the smallest way, for the others point of view to be taught. Parents must present a united front, after all, or the children will take over!
Even as an adult, I wish that my parents and I shared the same cult. I can't consult them on a lot of faith issues, simply because we're coming from two different starting points.