The largest issue here, from what I can tell, seems to be that your player is unwilling to accept anything outside of their current worldview, or paradigm.

Regardless of what RPG system you use (at least almost, I'm sure there are exceptions), you're going to run into situations where the game's reality is fundamentally different from ours in some way.

One of the great things about RPGs is they can challenge our worldviews, and allow us to explore realities different than our own. If your player isn't interested in exploring that, then maybe RPGs aren't their kind of game.

You have to consider the ramifications of other aspects here as well, like the concept of undead, resurrection, demons, devils, or even the concepts of magic and casting spells. It doesn't seem like a big leap to think if your player has an issue with Paladins and polytheism, that there might be some other issues along the way.

The fact that your player seems unwilling to suspend disbelief and explore a reality different than their own doesn't bode well for a) playing a fantasy game, or b) playing such a game with different people that may behave unpredictably.

I haven't dealt with this explicit situation where a religious player is unwilling to adapt, and I imagine there are only a few of us on this site that have, but I've encountered plenty of other scenarios where people get in the mindset of "I'm right and you can't tell me otherwise, even though the situation calls for it."

The best you can do is include them and encourage them to "play pretend" with their friends for a couple hours. Remind them of classic fantasy series like C.S. Lewis's The Chronicles of Narnia or of J.R.R. Tolkein's The Lord of the Rings. Both of those authors had strong religious beliefs yet were able to translate them to fantasy settings.

The long and short of it is, there's no good way to "convince" your player to play D&D and there's no good way to sculpt D&D to their ideals. It's all about perspective, and hopefully your player can come around to the idea that this is really just pretend.

If not, then the answer may be that you just don't play RPGs with them.

One player should not be allowed to force his personal views on the other players. If he has a problem with Paladins, he's not going to react well to Clerics either.

This player needs to realize that the game world is not the real world, and it does not need to conform to his ideas of how the real world works. There are other stories - Greek and Roman, alongside a broad number of non-D&D fantasy works - that feature pantheism.

Unfortunately, if your player is vehemently opposed to the very concept of pantheism, you probably won't be able to convince him otherwise. Your best bet may be to remove him from the game. For the health of the game and the integrity of the teaching experience, it's best to do it before play has begun, rather than try to eject him later.

I really don't know how to deal with this, because I would prefer not to limit the classes or my world creation because of this

You have a couple of options:

• Dungeon Master's Guide has a variant for a monotheistic setting (at least in 5th edition). You can use it, if it works for you.


• Don't touch the religion topic at all. If your setting requires polytheism, don't call them "gods", call them "powerful entities". Yes, some people pray to them.


• Distinguish player's beliefs and character's beliefs. It's a game, where you're supposed to kill people and loot their bodies. I don't think you do the same things in real life.

Whatever approach you take, you need to make it clear to this player that they hold no veto over other players' choices. (thanks @frog)

Have an open discussion with your player on the type of game D&D is and the setting you will be playing in.

A single player shouldn't be dictating what classes everyone at the table can and can't play, and you will need to talk to your player about this. Here's my advice on how you might want to shape that discussion:

• 
  Do not criticize your player's religious beliefs. - Doing so will most likely cause them to become defensive, shut down, and close the door to further conversation. Don't approach this as a problem stemming from the player's religion (because it probably isn't), but one coming from the fact that they don't understand the game yet.


• 
  Try to find some common ground. - Point out the ways in which D&D, and particularly the setting that you are playing in, departs from your own beliefs (religious or otherwise). Explain to the player why playing the game doesn't bother you despite those things. Maybe you don't believe in ghosts, zombies, or magic, but they all exist in your setting anyway. Tell your player about how they add to the adventure without really challenging your beliefs; it's a fantasy game, after all.


• 
  Explain to your player that D&D takes place in a fantasy setting that is not a reflection of reality. Try giving your player a quick description of your setting, emphasizing the ways in which it departs from reality. Is your player ok with a world that contains Elf Wizards, Gnome Sorcerers, Owlbears, and Flail Snails, but not one that contains multiple gods? Try asking them why that might be the case. Is the one any more fantastical than the other? Pointing out some of the more ridiculous things in the game may help make the point with a bit of humor.


• 
  Make sure your player understands the difference between roleplaying and real belief. Remind your player that the beliefs of the character don't always align with the beliefs of the player. I've had atheist friends take on the roles of zealous Paladins of Bahamut. Did those players actually believe that there is a Platinum Dragon living on the sacred Mount Celestia? Of course not, but they enjoyed playing the game in that role. Making a comparison to actors on a stage or in a movie may help. An actor in the role of Shakespeare's Macbeth might well portray the role of a hero descending into a power-hungry tyrant, but this is not a reflection of the actor's character or beliefs.

Hopefully after a conversation, your player will come to realize that just as the just as D&D doesn't adhere to the laws of physics, it doesn't necessarily adhere to spiritual laws either. That departure from reality can be a big part of the fun if everyone can get on board.

If your player doesn't see your point of view after a discussion, or understands but simply decides that this doesn't sound like the game for them, that's ok too. Maybe they can create a different setting or find a different role-playing game that fits them better and they can DM a game for you.

Hopefully this helps give you an idea of how to approach this conversation.

You'll Need to have a Very Frank Chat with your Friend

There are plenty of ways you could compromise so that HIS character can worship the "one true God"- he can even try to persuade his fellow players to convert (in character, of course). What he CANNOT do is dictate what content you can put in the world, or what choices other players make for their characters. Doing so not only undermines the other players' enjoyment of the game, it makes it so you cannot do your job as DM effectively. If your friend cannot agree to that, it's perfectly OK to politely suggest that D&D might not be a good fit for him.

Compromise is still Important

Having said that, tailoring your campaign to suit your players is something all good DMs should do. You need to know your players and create an environment where they can all enjoy themselves, and just as you'd avoid creating a role-playing scenario around a real-life trauma you know a friend has suffered, you should be aware if your campaign is going to make one or more of your players really uncomfortable, and try to find a way around it

A Possible Compromise You Might Consider

Perhaps your friend's character practices a faith that believes that the "gods" of different pantheons are all actually aspects of the "one true god". In a last ditch effort to save all of creation, the "one true god" sacrificed itself millennia ago- but since it couldn't be destroyed, it was instead shattered into fragments people now draw power from. His character is trying to live a life devoted to that one true god, in a world that has forgotten the origins of the "gods" they follow and fight over.

For your friend, this could be compared to role-playing what life may have been like for early Christians in the Roman era, when the majority of the population had a different set of beliefs, and his faith was potentially dangerous. Most importantly, he'll be role-playing living in a world where no one feels it is necessary to concede to his world view.

Be Prepared, Though

Still, I have to agree with G. Moylan's answer that the objections of the player in question are unlikely to stop at polytheism- you're going to have push-back the first time you introduce the undead, or magic, or any number of other things. 5e is moving in a very socially progressive direction these days, so you'll probably need to decide early on whether you want to embrace same-sex marriages, gender-changing elves and the like.

Problem: the player doesn't understand player and DM roles

This may not be solvable. And it may be. Don't play until this is settled.

I tried to explain that it's a fictional universe in which there are
different gods, to which the player responded, "No."

Your first answer to this is "Yes, it is, and if you don't accept that, then you are unable to play in this game." There are two reasons for this position as the way forward if you want them included in a D&D 5e game. (But I am not sure that this is about the game in the first place ... we'll save that for later).

1. This particular game is structurally built to be an exploration of a subset of the lands of make believe, with magic and strange things that are not in this world.



2. 
   

   The DM is in charge of the world and that cosmos (to include such deites as the cosmos has) and - this is the key point - the DM's rule on that is the last word. See the DMG sections on "Master of Worlds" and "Master of Rules" in the opening chapter for more detail (pages 3-5). You need to brush up on your understanding of this system for your own benefit.

   
   
   

   There are a wide variety of role playing games where the world building itself, from the ground up, is a collaborative exercise. D&D 5e isn't one of them as laid out in the DMG at the structural level of world building.

   
   

There are a variety of ways that players in D&D 5e contribute to world building.

Each table will have a variation on how much the players flesh the world out.

• 
  

  I've seen it go from nothing, to the point in my brother's campaign
  where I recommended we add a deity, outlined its portfolio, and my
  brother, the DM, approved it.

  
  
  

  And we've seen all points in between.

  
  


• 
  

  In my first 5e campaign, I didn't find a deity that I liked in
  Faerun, so I worked with the DM and asked about some options. We
  settled on Thor. Notice that I didn't say "NO, ONLY THOR!" I worked
  with the DM to come up with a deal that we both liked. (With your player,
  see below).

  
  
  

  Telling you who the deities are or are not is beyond the player's level of contribution unless you (per my examples above) are amenable to a change in the deity line up, and for that matter, your other players are as well as interested parties.

  
  
  

  1. If you are playing in the baseline "Forgotten Realms" setting, the
     deities are listed in the PHB in Appendix B. So too are a variety
     of other pantheons such as Dragonlance, Eberron, Greyhask, as well as Celtic, Egyptian, Greek, Norse, etc).

  
  

  2. If you are playing in your own world, whatever that world works like
     is how it works, deities or no. See the DMG for detailed treatment of that.

  
  

Sit down with this player and show them that you are the Master of Worlds per the DMG. It's in writing. The DMG is your support in this case, but this player apparently needs more than your say so. (See below for how this may be a symptom of a bigger problem).

If they still do not accept this premise, then D&D 5e isn't the game you ought to be using if you want this player to be included. They are toxic and will be a problem for your whole group.

It may be best to pick another game

For raw world building you can try Microscope. Some of the veteran players here have used Microscope as a prelude game to get an idea on what the world will look like before they embark on an adventure game in "another world" or in another time and place . There are numerous other games where the GM role isn't spelled out as having so much say in what the cosmos looks like. Those may be a better fit for your group if you really want to have this player included.

Lastly, you are being pushed: this has begun as a test of wills

Your player obviously has strong feelings, and may have a very strong personality. It may be that, when it comes to a test of wills IRL, they may be less willing to compromise than you are. If that is the relationship between you two, you've got an interpersonal issue to deal with that may not fit with playing RPGs together.

A rule learned by hard experience over the years: bad gaming is not better than no gaming.

If this kind of pushy behavior is to be expected session after session, there is no point in embarking on the game with this player included. I've seen games broken up, and in a couple of cases, friendships ruined, all thanks to disputes over a game. Yeah, it's an awful experience. An RPG isn't worth ruining a friendship over. (In my opinion and from bitter experience.)

"I'm already invested in 5e and playing that, so I also don't want recommendations for other systems"

It's your life: unless you get some movement on this player's part, you are setting yourself up for a hard time. If you want a train wreck, you can ignore what experience has taught me. Find out how the other players feel about this: actively solicit their input. Spend the time and effort to build a consensus. You need the buy in of all players, or you'll be wasting your efforts as a DM.

We play these games to have fun, not to get into an argument each time play starts.