Mmmm... I could not disagree more on that view. A fighter knows he's a fighter.
How? Can he see his own character sheet?
They know what they can
do. They don't know their game mechanics.
They know that they can
fight, so they can
describe themselves as a 'fighter'. But so can barbarians, paladins, rogues.... What they
can't do is look at their own character sheet and see how many levels in what D&D classes they have.
The paladin class was inspired and named after The Twelve Peers of Charlemagne, known as Paladins. It is of course feasible that there is an in-game organisation that calls itself and its members 'paladins'. But what would qualify a person for membership? Having levels in the class? The people can't read the character sheet!
Okay, we know paladins can use heavy armour, martial weapons, and cast divine magic spells. They can swear an oath. But a Ftr/Clr can do all these things in game. Use heavy armour and martial weapons, cast divine spells. Swear whatever oath their voice is capable of.
Ah, but can they do extra radiant damage with their weapons?
Er, some can, yes. Some can do an extra 1d8 radiant on a successful weapon strike.
So what thing, discernible to creatures in game, do these in-game paladins have to do to qualify? I'm pretty sure that whatever it is can be done by non-paladins. And should be no problem for multi-class paladins! Taking a level in another class doesn't stop them being paladins anymore!
Would not work this way. A player can learn the class of another player since the player would be "teaching" his way to his friend. But if no divine caster are in the group, then the fighter better have a good story to justify this. With the right set of background it could be entirely possible. A fighter taking the acolyte background would be justified in doing so, especialy if he had been RPing the religious aspect of his fighter. I would allow and encourage such a change.
But the fighter with the criminal background deciding somehting like this out of the blue, without any previous RP for such a switch, will have a lot explaining how it could happen (storywise). And I would not be the one asking for the motivation (RP wise) for this kind of switch. I can bet you that it would be the other players that would ask for what was the trigger. Not only I would not allow it, but the other players would expect as much.
As for your experience in multiclassing. You are one of the few that do it this way. IME most multiclassing happen when the character starts higher than 1st level. I may be wrong, but that is what I have seen so far not only in my games, but in the games of a dozen or so masters that are in my area (at least those that I know and have as accointance).
I
usually, not always but usually, plan a large part of my multiclass at the beginning. For example, my concept is for a multiclass Ftr/Wiz, and if I was playing 1e or 2e then I'd start the game as a Ftr 1/MU 1, but in 3e/5e the same concept means that I have to be 2nd level before I can actually
use both classes. That doesn't change the concept though. I already have the seeds of
all my future classes as part of my pre-first level training, and gradually become experienced enough and get enough practice to develop my abilities as I level up,
in exactly the same way as a single class character does.