Arial Black
Adventurer
You bring up some interesting points, and I'd like to address each in turn with a contrasting view:-
In 5e (and since 3e), there are no 1st level MC characters. This is because of how the game mechanics work: you get one class level per character level, therefore if you only have one character level you can only have one class.
Up until then, you either started as a 1st level single class, or as a 1st/1st multi-class, or as a 1st/1st/1st multi-class, and stayed that combination of classes that each got improved relatively equally ('relatively' owing to the different XP tables for each class). The only exception was the (amazingly rare) dual-classed human.
Oh, and BECMI had 'elf' and 'dwarf' as classes, where 'elf' was really a fighter/mage.
But, they are all D&D. We can expect to create our PCs, using the rules, and craft their story around that resulting character sheet.
So, I could be making a PC for BECMI or 1e or 2e. Let's say that, mechanically, he is an elf trained in the arts of magic and war.
* in 1e, fighter/magic-user 1/1
* in 2e, fighter/mage 1/1
* in BECMI, elf 1
But it is the same character concept each time: an elf trained in magic and fighting for literally at least 50 years before turning up as a level 1 PC. At least as compelling a justification for his 1st level abilities as any single class wizard or paladin.
Great. Now I'll use the same character concept and make a PC in 3.5/PF/5e. If I get to 20th level, I might end up 10/10, or maybe 11/9 or 12/8 or something, but not a 'dip'.
We already have decades of D&D experience that this is a perfectly valid concept. There is no reason to forbid it!
We also know that, at level 1, he won't be multi-class yet; I have to wait until level 2 minimum. This is not a problem with my concept! This is simply a consequence of the way the levelling rules work!
So the objections I see on this forum on the lines of "It takes years to learn to be a level 1 wizard! It doesn't make sense to suddenly gain your 1st level of wizard overnight!" are easily rebutted. The problem here is not that I didn't put in the years of study: I put in 50 years, thank you very much! The problem is that I can only actually use the ability of one of my two classes until I've killed 300xp-worth of goblins!
Let's say you are a paladin, totally dedicated to both you god and your oath. Now let's say you take a level of fighter, or 'dip'...something. "Oh, no!" I hear you cry. "You've abandoned your god and your oath! You've stopped being a paladin!"
Rubbish! I'm still a paladin, still loyal and dedicated to my oath and my god. Learning a few combat tricks doesn't change that, developing abilities through my heritage doesn't change my faith, the fact that (unknown to me) my parents bargained my soul to a fiend before I was even born before they abandoned me on the steps of the temple doesn't prevent my from using my paladin abilities in any way. Certainly not rules-wise!
So DMs are literally inventing ways to nerf your character choices, by making up increasing desperate 'reasons': MC PCs don't stop being one class when they take a second. Maybe you cannot worship two gods, but there's nothing stopping one patron from trying to subtly corrupt the agent of a good god. Will he be corrupted? See next week's exciting episode! That Pal/War is not an invalid character concept, it's a great one!
Weren't you listening to a word I just said!
Or maybe I could have made wiser choices while quoting.
But I addressed this already.
See! All very reasonable. I just wish more DMs were as reasonable, and fewer resorted to spurious 'reasons' to reject a concept before even listening to your idea.
I don't necessarily dislike multiclassing, and have used it for decades. I'm not a fan of "dipping" or designing 20 levels of character with all of the multi-classing planned out before the character ever sees play either. The mechanical aspects of multiclassing can sometimes be a bit of a challenge. For example, 1st level characters get a bunch of things right away. Thematically, that makes sense. They've been training for several years for their new avocation. But taking it as a second class doesn't provide that. In 3/3.5e for example, you would be older when you gained 1st level in certain classes, since it theoretically took longer to learn them.
In 5e (and since 3e), there are no 1st level MC characters. This is because of how the game mechanics work: you get one class level per character level, therefore if you only have one character level you can only have one class.
Up until then, you either started as a 1st level single class, or as a 1st/1st multi-class, or as a 1st/1st/1st multi-class, and stayed that combination of classes that each got improved relatively equally ('relatively' owing to the different XP tables for each class). The only exception was the (amazingly rare) dual-classed human.
Oh, and BECMI had 'elf' and 'dwarf' as classes, where 'elf' was really a fighter/mage.
But, they are all D&D. We can expect to create our PCs, using the rules, and craft their story around that resulting character sheet.
So, I could be making a PC for BECMI or 1e or 2e. Let's say that, mechanically, he is an elf trained in the arts of magic and war.
* in 1e, fighter/magic-user 1/1
* in 2e, fighter/mage 1/1
* in BECMI, elf 1
But it is the same character concept each time: an elf trained in magic and fighting for literally at least 50 years before turning up as a level 1 PC. At least as compelling a justification for his 1st level abilities as any single class wizard or paladin.
Great. Now I'll use the same character concept and make a PC in 3.5/PF/5e. If I get to 20th level, I might end up 10/10, or maybe 11/9 or 12/8 or something, but not a 'dip'.
We already have decades of D&D experience that this is a perfectly valid concept. There is no reason to forbid it!
We also know that, at level 1, he won't be multi-class yet; I have to wait until level 2 minimum. This is not a problem with my concept! This is simply a consequence of the way the levelling rules work!
So the objections I see on this forum on the lines of "It takes years to learn to be a level 1 wizard! It doesn't make sense to suddenly gain your 1st level of wizard overnight!" are easily rebutted. The problem here is not that I didn't put in the years of study: I put in 50 years, thank you very much! The problem is that I can only actually use the ability of one of my two classes until I've killed 300xp-worth of goblins!
In a world where the gods are known to be real, may have actually walked the planet, and that they grant very real magic and abilities, I think they would be that more important. What separates a layman from a cleric or paladin? Faith. Much stronger faith, and a much stronger commitment. If you're dedicating your entire life to your deity to the degree that they are imbuing you with their power, why/how would you ever change? Would switching to a fighter be leading you on that path? If your worshipping a god of war, perhaps. And that makes sense. But if you're worshipping a god of peace, or nature, or knowledge, etc., not so much.
Let's say you are a paladin, totally dedicated to both you god and your oath. Now let's say you take a level of fighter, or 'dip'...something. "Oh, no!" I hear you cry. "You've abandoned your god and your oath! You've stopped being a paladin!"
Rubbish! I'm still a paladin, still loyal and dedicated to my oath and my god. Learning a few combat tricks doesn't change that, developing abilities through my heritage doesn't change my faith, the fact that (unknown to me) my parents bargained my soul to a fiend before I was even born before they abandoned me on the steps of the temple doesn't prevent my from using my paladin abilities in any way. Certainly not rules-wise!
So DMs are literally inventing ways to nerf your character choices, by making up increasing desperate 'reasons': MC PCs don't stop being one class when they take a second. Maybe you cannot worship two gods, but there's nothing stopping one patron from trying to subtly corrupt the agent of a good god. Will he be corrupted? See next week's exciting episode! That Pal/War is not an invalid character concept, it's a great one!
Switching to warlock, where you are making a pact with another being for power? I think that's a pretty good recipe for being an x-level ex-cleric/1st-level warlock. So for me, divine classes are more than just learning a vocation. They are about fully devoting your living being to their cause.
Weren't you listening to a word I just said!
Or maybe I could have made wiser choices while quoting.

Really what it comes down to, is that if you are multiclassing out of a cleric or paladin, then there needs to be some good justification to make it work. If you can provide that, no problem, as long as you maintain the tenets of your deity. Multiclassing in is no problem as religious conversions are not only a thing, but desired by the deities.
Warlocks, on the other hand, are quite different. Yes, they are making a pact with another being. But that other being probably doesn't give a crap if you use that power or not. If it's the traditional "selling your soul to a fiend" approach, then hey, they own you whether you choose to utilize their power or not. So I don't really care if you multiclass from that.
I'm not really concerned about tradition. I'm concerned about engaging with the setting. If you have an interesting idea and it works within the setting, then I don't have an issue with it. If you do have ideas of what you want to do in the future, I don't really have a problem with that, but will recommend you try to not let those plans/expectations prevent your character from growing in a different direction if that seems appropriate. Many of us have had plans for our future, and yet find that we end up in very different places than we expected. I don't really forbid any specific combination, and expect that there will be some reason for it in the narrative of the character's life. But that has to fit with the setting as well.
See! All very reasonable. I just wish more DMs were as reasonable, and fewer resorted to spurious 'reasons' to reject a concept before even listening to your idea.