So first of all, I really like this idea. I tried it once or twice a long time ago and I'm still fantasizing about it. With that out of the way, let's move to my 2 cents.
Make the NPCs well-rounded characters
These NPCs aren't just some recurring characters, not to mention some one-time ones. They're gonna be with the party for a very long time. Whole adventures, actually, which is quite a lot. This means that they should be really well rounded. Your players should get to the point of distinguishing between them after just a few words, especially when the NPCs are the ones who spark the conversation.
More than that, though, they should be more than cardstock characters. You want them to express feelings, you want them to have goals, and you want them to react. Otherwise, your players will have hard time connecting with them, and this is far less good. As a rule of thumb, make them round and distinguishable from each other. A wizard is different from a fighter, true, but we want a greater difference. The prince will have hard time adjusting to the wilderness and to the bugs in his bed. The merchant will always stop to collect the better loot so she'll be able to earn much from those bargains. Classes are a dirty way to distinguish, but they're better when accompanied by some other traits.
Don't let them speak right one after the other
The players aren't coming for the game as an audience for a theater, they're here to play, and hearing NPCs talk with each other is far less fun. If you must make them speak between themselves, make it as quickly as you can and immediately move on. If you can narrate the conversation instead ("they're talking about what happened, Elsa thinks that they should go east and Hans thinks that they should go west…"), it is far far better.
Give the players the center stage
The players and their characters are the real stars of the campaign. The NPCs are extras, and should always be seen like for you that when you're playing them. If they're far cooler than the PCs, or if they have much more screen time, something is off with your campaign. It's better to not have the NPCs with the group than to let them illuminate and shadow the PCs.
They're not all knowing
Think for yourself what is more important, a character who knows everything or a character who knows only part of them. For me, it is the latter. That comes from one simple thing: There's no drama when everyone knows everything (it is not entirely true, but that's for another time). Make them say sometimes stupid or idiotic things, make them come to wrong conclusions, let them make mistakes. They're not a kind of supernatural deity who knows everything, but humanoids who are as humane as the characters, and they should be played this way.
Make them important for the story
They should always be important to the overall story, in one way or another. In one of my more successful D&D campaigns, the characters had to escort a princess to a neighboring kingdom through the forests. Having to keep her safe from one hand, and dealing with all of her complaints from the other one made the game so much richer. They don't have to be important to each and every one of the scenes, but they should always be important for the overall story. Otherwise, the characters may just leave them to rot one day, when things will turn the wrong way.
And an end
Hope I succeeded with helping you a little bit.
Well, this is a great question, and I must say beforehand that my experience with an organization of villains is pretty narrow. But I do hope that what I'll write here might be of some use to you.
Inspiration
One of the key things that you can do is to find inspiration for your villains in other sources. I once based an organization of villains around the 7 exes from Scott Pilgrim vs the World. Another time it was around a bunch of villains from westerns. The trick is to find some villains that are cool and draw inspiration from them.
This inspiration can be in terms of copying the villains, or it can be in terms of taking certain parts of their personalities. It can even mean analyzing what made them so memorable and using that someway.
Make them somewhat related to the characters
You want your players to care for them, to remember them, or something around those lines. For that, you have to make them related somehow to the PCs. While having them compete for the same goal is nice, I do prefer something closer to the trick utilized in Scott Pilgrim, having them the exes of a certain friend, relative or the like. Maybe have some of them be immediate family of the PCs? A true connection is far better in order to make the players care for them enough to hate them, love them, or even love to hate them.
Make each one of them unique
This is also very important. While they are all a part of an evil organization, they must be unique by themselves. Each and every one of them is also a villain that the party will have to encounter one day or the other. In Hot Fuzz each one of the conspiracy members is unique and is colorful enough to be remembered. We have the couple who hate the reporter for stating the wife's age is higher than it really is. We have the farmer whose mother has a rifle. We have the chief of police who doesn't punish criminals and we have the manager of the Supermarket who is always there when you least expect him. In Scott Pilgrim we have the Female ex who Ramone experimented with, we have the actor who has doubles, we have the leader who is an agent and who can control Ramona and so many more.
Go for it too, make them unique, make them colorful, make them come alive as persons and not only is a part of an organization. The fact The Joker fights sometimes alongside with Bane doesn't make each one of 'em less frightening, unique or evil, right? Why should that be the case here?
Have something common for all of them
Being unique doesn't mean that they have to be completely different. In Scott each of the exes is an ex of Ramona, and an idiot. In Angel they are all attorneys. In the first and second seasons of Buffy they're all vamps. Have something that unites the villains that is common to all of them. Maybe all of them have similar clothes or use the same perfume? Maybe they all talk a bit funny?
As a bonus, if you can make the uniting thing a thing that is cool and all by itself, which can make alone the villain stand out, it is even better.
Let the players know of them in advance
The characters should know about the organization pretty early, but having the players know about the organization even earlier can do magic sometimes to your campaign. If they'll know that their characters will fight an organization of villains, the players will look for them (and with far greater anticipation). Always remember that the players are authors as well as audience, and if they will be looking forward to meeting the villains they will far better enjoy this meeting between their characters and villains. But they will also search for them; lead their characters to them, and so much more.
Addendum
I also highly recommend reading answers to questions like these two. While they do center on creating a single villain, they are still quite useful and in more than just creating and fleshing each and every one of the villains in the organization.
Best Answer
Treat the NPC as you would a Monster
There isn't a direct equivalent for a 25th level cleric correlated to CR, so you need to go through the "how to build a monster" steps from the DMG pages 273 - 283. Likewise, look at "Monsters with Classes" on page 283 of the DMG. It's not an exact science, though the guidance on page 282 says this:
That means that you would use the tables on pages 274 and 275, as well as features on pages 280 - 282. It is going to take some fiddling to get the CR "just right" when you combine hit points, offense, defense, spell casting, etc. When you consider that an Empyrean(MM, p. 130) is CR 23, some of its features (like spell resistance) could be earned after level 20 by a PC via Epic Boons (DMG p. 232).
An issue you need to address is the "bag of hit points" feature that increasing CR brings with it. AD&D 2e characters don't typically have 300+ HP. How do you render the character's HP as "effective hit points" - immunities, resistances, and Armor Class are some ways. The AC value in the table tops out at 19, yet many high CR monsters have higher AC than that. These are all ways the NPC avoids getting hit. You can also significantly boost the offensive CR of the NPC. (The Empyrean's spell save DC is 23; see below for more on that).
Alternatively, this is a PC run by the DM; translate based on stats/items
The DMG suggests building an NPC like a PC (DMG p. 282), which points you to the PHB. Start with a level 20 Cleric, and then add magic items, blessings(p. 227-228) and epic boons (DMG p. 230-232) to reflect increased level, capability and challenge. Since the Cleric is a spell caster, boosting the Cleric's wisdom so that save DC's are higher than 20 (table, DMG, p. 274) can render some spells as an "auto success" against players with no proficiency in that save (based on ability scores). That has an impact on the Offensive CR, which you'd average with Defensive CR to get your first estimation of CR. Tinker with this, since Save DC boosts can wander into "save or suck" territory easily.
Example: A proficiency bonus for a 22 CR creatures is +7. With a Wisdom of 24 (boons/blessings) your cleric would have a save DC base of 22 (8+ 7 + 7) There is no auto success on a saving throw - any PC/creature without proficiency in Wisdom saving throws fails the save Hold Person unless their wisdom is at least 14 (+2 bonus) or they have some other magical boost to their save is applied (blessing, ring of protection, etc). Advantage won't help. Save DC of 22 is typically seen in a creature of DC 27, and would contribute to this NPC cleric's offensive challenge rating. (DMG p. 275).
Fiddle with that - the cleric's Save DC, via either stats or magic items - as you combine Offensive and Defensive CR to find your sweet spot.
Consider adding Legendary Actions and / or Lair Actions
Using the Empyrean again as an example, add some legendary actions to this cleric, just as the NPC Warlord (Volo's Guide to Monsters) emulates a high level fighter, but adds legendary actions to make him tougher.
Consider just the extra attack legendary action for your NPC high CR cleric. The cleric casts a spell, and then casts another spell as a legendary action. That's quite a bit more powerful than the typical action economy's restriction to one spell per turn for spell casters.
You can also, if you want this cleric to be the BBEG, add Lair Actions/Features to this NPC to ramp up the challenge. A cleric of that high a level is sure to have a base/temple/shrine/chapel/something, so make it a source of power the way a dragon's lair is.
Character details would be helpful here ...
Without posting the details of the character in question, a more detailed answer can't be offered, but the above process ought to serve you in finding a CR that fits a level this 2e level 25 character.
Single monsters or a BBEG with Henchmen?
You may be better off in setting up the challenge such that your 20+ level cleric has some powerful allies (high CR demons or Celestials) such that your party cannot focus fire on a single target.