The natural religion gives rational sanction to all the demands of morality. Suppose reason directly tells us that it is good to subordinate the flesh to the spirit, that it is good to help others and to recognize the rights of other people like our own. Now in order to obey these demands of reason, one must believe in reason--believe that the good it requires from us is not a subjective illusion, but has real grounds and expresses the truth, and that that truth 'is great and overcomes.' Not to have this faith is to disbelieve that one's own existence has a meaning--is to renounce the dignity of a rational being.
Vladimir Soloviev, The Justification of the Good, von Peters, ed., Catholic Resources (Chattanooga, TN: 2015).