Religious duty consists in doing justice and loving mercy

Many deists affirm that religious duty consists in doing justice and loving mercy, that the heart of true religion is right conduct toward one's fellow human beings rather than creed, ritual, or ceremony. On this view a rational religion issues chiefly in morality: honesty, benevolence, fairness, and compassion. The formulation echoes the ancient prophetic call to do justice and love mercy, which deists could embrace as an expression of the natural moral law they held to be knowable by reason and binding on all.

The formulation directly echoes the prophet Micah's summary of true religious duty: "to do justice, and to love mercy, and to walk humbly with your God" (Micah 6:8), a text deists could read as confirming, from within scripture itself, that ethical conduct rather than ceremonial observance was religion's real substance. Benjamin Franklin, whose personal religious views leaned strongly deist, expressed a version of this ethic in his autobiography, describing his mature conviction that "the most acceptable service of God" was "doing good to man" rather than formal worship, and organising his own personal moral improvement scheme around a list of practical virtues rather than doctrinal creed, a characteristically deist reduction of religion to ethical self-cultivation.

This affirmation flows from the deist conviction that religion should rest on reason and nature rather than on revelation, ritual, or dogma, and that its proper fruit is virtue. It set deism against forms of religion the deists regarded as preoccupied with doctrine and ceremony at the expense of ethics. By locating true piety in just and merciful action, deism aligned religion closely with morality, and the belief is presented here as the ethical emphasis its adherents held. Ethan Allen, the Revolutionary War figure, published one of early America's first extended deist tracts, Reason: The Only Oracle of Man (1784), which met with considerable public hostility for its direct attack on revealed Christianity, an early sign that American deism, though influential among several Founders, was never a majority religious position even at its height.

As an organised religious current, deism declined through the nineteenth century, its rationalist and ethically centred impulse passing partly into Unitarianism, which retained a form of theism while further de-emphasising doctrine, and partly into the secular and humanist movements of the later nineteenth and twentieth centuries, so this deist emphasis on justice and mercy anticipates the very different, non-theistic humanist ethics that later kept much of deism's ethical emphasis intact.