As I see it, the sources of power for PCs are properly setting elements, and I have a strong preference for dissociating setting elements from the general rules of the game for more flexibility and customization.
I'm usually a DM. In my first 5E campaign, one young player (new to D&D) at my table wanted to play a melee warrior with magical powers. The eldritch knight didn't appeal. Instead, he played a dwarven tempest cleric with none of the religious aspects, and we had a great time. As a DM, I could have said, "No, the only way you can acquire clerical spell-casting is by devotion to a god and by promoting that god's interests." But what would be the point of that? He wasn't entitled, he wasn't trying to get away with anything, and I wasn't interested in preventing him from playing an exciting character. Where did the power come from? The player didn't know, the character didn't know, and none of us at the table cared, either. As a DM, I had a couple of explanations in my pocket in case it became relevant, but none of those explanations involved typical clerical relationships with the divine.
As a player, one of my favorite PCs (AD&D 1e) was a human fighter. I was up at about level 6 or 7, I think, when it felt that I had pretty well played out the character. After a little break, I decided to dual-class as a cleric. The PC was jaded, distrustful of the gods (and especially distrustful of his own understanding of the gods), and didn't believe he could atone for some terrible things he had done. When he started demonstrating clerical abilities, it shook him up and really changed the trajectory of the character. Never did figure out why he of all people was imbued with the power. The DM could have vetoed, of course, as that's not how the PHB describes how clerical abilities work. But I wasn't t entitled and I wasn't trying to get away with anything. I just had a character in mind that really interested me and re-engaged me in the long term game that had started to grow stale for me.
I am fully supportive of DMs developing campaign settings with limitations that bring focus to the tone and themes and atmosphere the DM wants to evoke. The rules, though, should provide a wider range of options than any specific campaign setting does. That's what makes D&D D&D for me. It's customizable, hackable, adaptable, re-skinnable, and all that flexibility is encouraged.