The Cleric is a militant priest -- Knights Templar or Knight Hospitaler. At least some of these orders took vows to avoid shedding blood. In true rules-lawyer fashion, creatively interpreted these vows in a way that allowed them to use bludgeoning weapons, thus the restriction on clerical weapons in earlier editions. In most ways, the cleric is priest first, but a priest that prays softly and carries a big stick. They are ordained, and have the rights and privileges of such. This interpretation screams for an NPC priest class, though, that's better at casting but not as good at fighting.
The Paladin is the "knight in shining armor". They're really a Fighter, but they are so pure of heart and full of faith that "the powers that be" bless them in a number of ways. If you're familiar with WoD or Ars Magica, a Paladin is just a Fighter with True Faith. Anything the Fighter gets that the Paladin doesn't is either for game balance or because the Fighter is more interested in winning than keeping his hands clean.
Druids are pagan priests that are, compared to the Cleric and Paladin, rural and wild. They could be animists (which is how I use them), but really make more sense as followers of the Old Gods. Most of the Druid abilities are to differentiate them from the Cleric. They get Reincarnate, rather than Raise Dead. They can't draw on the power of God to turn undead, but are resistant to scary things of the woods and can change shape.
The further away from the pseudo-Medeval, pseudo-Christian settings, the less sense the divisions make. That's why I tend to use Druids as animists and Clerics as god-followers. With the 5E green Paladin, it could be an exemplar of either.
I like that much better than making the Ranger the Druid-Paladin. The original niche of the Ranger was a Fighter who was focused on skirmishing and protecting the borders of civilization. They were "part" of civilization, if apart from it, and not barbarians or pagans. They learned spells because they were exposed to hidden lore and their survival depended on making some use of it -- which is why they had wizard spells in 1E. Personally, I always saw the Ranger as the Mage-Paladin, not the Druid-Paladin, but I digress.
Answer to the original question: The Cleric is tied to an implied setting that is no longer used. We may need something in that role, but the Cleric is a poor fit.