Paladin was this narrowly defined class, but clerics were also narrowly the mace swinging Templar/Hispitaler type of christian holy man. In 5e with oaths paladin is more of a champion of a cause. Arcana Unearthed / Arcana Evolved (from Monte Cook) has a champion class which is actually a much better version of the paladin concept than any TSR/WotC version ever was. The paladin has a clear role.
The cleric also has a clear role. And organized religion is all around the world. But the questing / adventuring cleric has next to no literaly backup. The holy man, seers, diviners, healers mostly stay in one place, and the protagonist(s) visit them if needed. They could be key to the story, but rarely go out to do the stuff directly.