I don't see clerics as knights at all, i see Clerics as just priests with different specialities dependingnon deity and domain. Some can fight, others focus on orisions(cantrips), some more on skills and so on.
The clerics are seen by most as just priests, what ever knightly connection they had in the past has drifted away.
From PHB p 20 (author Gary Gygax):Clerics are clergy. Paladins are laity. Paladins do not have and can not hold ecclesiastical rank or authority. They can not perform religious rituals or sacraments. They are, instead instruments who receive patronage from divinity.
This class of character [the cleric] bears a certain resemblance to religous orders of knighthood of medieval times.
Which is to say, clerics as conceived of in AD&D weren't particularly ecclesiastical - given that the crusading knights were lay brothers, not clergy. If one took level titles seriously then a cleric was ordained at 3rd level ("priest").
There's also no reason to think that paladin's cannot hold ecclesiastical rank or authority. Kings like Edward the Confessor or (the mythical) Arthur certainly exercise ecclesiasitcal authority eg they appointed bishops within their realms. Charlemagne (perhaps not a paladin, but nevertheless a revered figure who sits in the general space of the archetype) was annointed Holy Roman Emperor by the Pope, and also appointed bishops in his realm.
It is possible, of course, to construct differences if one wants to - though this assumes a contrast between church and laity which may have no currency in some gameworlds - but the original archetypes were the same.