What is the difference between Diesm and Uniterism both then and now in relation to David Hume's Dialogues Concerning Natural Religion?
I think there is a double and significant spelling error in your question. I think you mean "Unitarianism," a 17th century development in Protestant religion that followed upon the Reformation. In contrast, "Unitarism" is an industrial relations management theory innovated by Oxford's Alan Fox that holds that there are three organizational frames of reference: unitarism, the association of individuals in social [employment] membership that exists to "satisfy common interests"; pluralism, the association of individuals in memberships that exists to "satisfy the interests of separate but interdependent groups"; radicalism, an illegitimate relationship with no mutual membership that exploits and exists to "satisfy interests of the dominant party" (Conor Cradden "Untarism, Pluralism, Radicalism ... and the Rest ...?" Université de Genève)
I feel fairly sure you mean the misspelling of the religious perspective "Unitarianism." Unitarianism is a religious perspective that originally accepted the existence of God but viewed him as a Unified, single God rather than a Trinitarian God (three manifestations of one God). Now, belief in God is not a standard feature of Unitarianism as members hold beliefs through a broad range of ideas including humanism and agnosticism. Unitarianism has some commonality in that both disdain separation of people and religious ideologies based upon differences and both advocate a commonality of religious belief tolerance.
Deism is belief in a higher intelligence, called God, that is the original cause behind the world as we experience it and behind the moral order that governs Western thought. Deism relies upon reason and natural laws to know the definition and attributes of God. Deists believe in the unity of the Intelligence behind the original cause and that this Intelligence is common to all cultures, ethnicities, peoples; thus the divisions based upon religion, sect and beliefs are artificial and without foundation.