Cult effects culture. A people cannot efficiently coordinate their activities except insofar as they share a common understanding of the way things are, and of the proper way to deal with them. At the very least, they must agree about what is real, what reality is like, what it is for, and so forth; they must agree about First Things, and indeed Most Things. This they generally do, without ever even noticing all their myriad agreements; men rather tend to notice only their irksome disagreements, however petty.
Thus to cohere, a culture must recur to its common cult, and must rehearse it together. So is there always an established religion.
A people among whom heterodoxy regarding First Things begins to gain a foothold begins ipso facto to become confused in their motions: in their heads, hearts, and acts. Their loyalties are then divided, and so vitiated, at least at the margins.
Heterodoxy is cold civil war. Let it compound long enough, and it will go hot. So healthy societies must control for heterodoxy, especially about First Things.