The mountains and the canyons started to tremble and shake
as the children of the sun began to awake.
It's not easy pulling off a large mono-theistic religion in a FRPG, especially one where the world not only has a lot of pantheons, but also where all these goods are assumed to be real.
Fortunately there are plenty of examples from the real world. Especially in the Christ-like figure of Mithra or Mitra and the similar Sol Invictus.
I think a god like Mithra would be interesting in D&D.
The Church of Sol Invictus
In my Mystoerth world the Church of Sol Invictus seeks to unite the faiths of the world under the banner of one Sun God. They have a lot of influence in the temples of Pholtus, Pelor, St. Cthubert and Ixion. There are many similarities to this faith and the one of Taiia in 3rd Edition.
Proponents of Sol Invictus believe that their god is the only true god and all others are false gods or even worse elevated demons or mortals.
But the Church of Sol Invictus has a number of other, deeper secrets.
In addition to only recognizing the sun gods of various faiths as the only true god, they believe that these gods are really only aspects of a much greater god.
The high priests and the leader of the faith, known as "His Brilliance", feel that these aspects are in fact aspects of the god known as "He Who Was".
He Who Was was an ancient powerful god whose chief lieutenant was Asmodeus. At the behest of Tharizdun, Asmodeus tricked and betrayed this god. He was ambushed by Orcus, Demogorgon and a third demon whose name has been erased. He Who Was managed to kill this other demon and even split Demogorgon into two. Demogorgon later regenerated into his current form, each half of his head regenerating in to two complete heads. Orcus killed He Who Was and in a further act of desecration used his skull and spine as the the great Wand of Orcus.
The clergy of Sol Invictus believe then that by combining these aspects of the Sun God and destroying Orcus forever they can have their god reborn. They believe that this rebirth will make their god more powerful than he ever was.
In my larger world St. Aleena was also a follower of this philosophy and the Church of Law and Order in City of Dolmvay is just another aspect of this faith.
The major foes of this Church are followers of Orcus and pretty much any demonic cult. Also feared and mistrusted are the faiths that follow the Moon.
The Church of Sol Invictus are among the main witch-hunters in my world.
I am planning to work elements of this church into my current game, but not sure how much detail the players are actually going to get. The use a word my wife hates, I am going to let it grow "organically".
Some of this organic growth began when I first started pulling all this together for 4e. It was building the 4e version of Aleena that got this idea rolling again. I guess this is one of the reasons I have not let go of my 4e books. A lot of the fluff is still compatible with what I am doing in 5e.
Not everything is figured out though.
Given that I grew up in the 70s I would love to add some more left-over hippy shit to it all. I would love to add something like 12 houses to the faith, something along the lines of orders. Or 7 orders and break them down by color. The 7 orders actually fits better with some other things I had done in the past. Each order representing a different aspect of the faith. I had already figured out Red (Military might) and Blue (Knowledge/learning). It should not be too hard to come up with something else. Obviously Green could be related to growing things. Violet could be Guidance. Why? Well it brings in a group I had created separately that could work into this faith. Maybe they absorbed them. I could even have a "Black" for the secret police within the faith.
I will give this some thought. But yeah this could be fun.