6 Now, morality, in man, by the mere fact that it should establish harmony among so many dissimilar rights and duties, since it enters as an element into every human act, necessarily supposes God, and with God, religion, that sacred bond whose privilege is to unite, anteriorly to all other bonds, man to God. Indeed, the idea of morality signifies, above all, an order of dependence in regard to truth which is the light of the mind; in regard to good which is the object of the will; and without truth and good there is no morality worthy of the name. And what is the principal and essential truth, that from which all truth is derived? It is God. What, therefore, is the supreme good from which all other good proceeds? God. Finally, who is the creator and guardian of our reason, our will, our whole being, as well as the end of our life? God; always God. Since, therefore, religion is the interior and exterior expression of the dependence which, in justice, we owe to God, there follows a grave obligation. All citizens are bound to unite in maintaining in the nation true religious sentiment, and to defend it in case of need, if ever, despite the protestations of nature and of history, an atheistical school should set about banishing God from society, thereby surely annihilating the moral sense even in the depths of the human conscience. Among men who have not lost all notion of integrity there can exist no difference of opinion on this point.
Source: Au Milieu Des Sollicitudes (Vatican.va)