However, when I mean a "divine source" I am referring largely to the "power of faith" either coming directly from a god like in FR, or possibly from themselves as Eberron hints at (the existence of gods is not explicitly confirmed one way or another).
In Dark Sun, magic from a divine source just doesn't happen. That doesn't invalidate the cleric class, or even religion, but it does make a very specific form of magic unusable.
The Divine is important, and difficult to define.
D&D 5e still needs descriptions of a Divine power source that are egalitarian, tolerant, and inclusive.
Many (probably most?) human experiences of the sacred are of harmony, mutuality, sharing, neighborliness, cooperation, community, egalitarianism, freedom, partnership, and love. Free will and free choice. And respect for each human.
By extension, each humanoid.
Part of the solution is to focus on a community. A sacred community can be small and local or vast and international. It functions well when each member has equal value and equal say.
D&D 3e moreorless defined the Divine power source as the "power of faith". Faith in the sense of the "power of positive thinking" is effective. Likewise, faith is personal and empowers the freewill of each individual. At the same time, an uncritical "faith in faith" shades into existentialism, and can even end up meaningless, solipsistic, dehumanizing, and nihilistic.
In 5e, the sidebar in Xanathars focuses on something pragmatic: a "cosmic force" or a "philosophy". In other words, the experience of the Divine comes thru something that is plausibly fundamental to reality. So "life" as a force is necessary for human lives and experiences to exist. Similarly, "love" as an ethical philosophy can include the symbiosis between plants and animals, that makes existence possible. This emphasis on something pragmatic feels more realistic and reliable. It makes more sense in a setting.
But even the sidebar of Xanathars has problems: such as the unconscious use of the word "serve".
Servitude, slavery, hierarchy, obedience, punishment, coercion, subjugation, intolerance, dictatorship, imperialism, and "worship". This nonegalitarian conceptualization of the sacred, is ultimately Hellenistic polytheism (blending Greek, Persian, Mesopotamian, and Egyptian), via later Hellenized empires, including the brutal Roman Empires. Forms of monotheism also emerged from within this matrix of Hellenism. The word "worship" is literally a form of violent supremacism, imposing a religious worldview that is alien to many cultures.
Yes, there are experiences of the sacred that a person perceived as Master commanding Slave. Especially within a culture that practiced slavery! But the Divine is also so much more.
It is a high priority for D&D 5e to avoid an unconscious ethnocentric assumption of servitude, when describing the Divine power source.
Each community experiences the sacred in its own unique ways.
D&D does well to be more inclusive.