The cleric is one of the most interesting additions to the original edition of DnD.
In many ways the militant holy man is a bit of an odd fit among the general swords & sorcery genre that informed the early development of the game. There is really nothing quite like it among the works of Edgar Rice Burroughs, Fritz Leiber, Robert E. Howard, JRR Tolkien and so on. Even CS Lewis doesn't have anything resembling the Cleric anywhere in the Chronicles of Narnia.
We have to dig back into history and lore to find warrior monks. Friar Tuck, the orders of Knighthood like the Templars, and further back perhaps to Old Testament times with some of the Judges of Israel. The most likely inspiration was Dr. Van Helsing, particularly as he was portrayed in the Hammer Horror films that Gygax and Arneson enjoyed so much.
While you could argue that the entire booklet Gods, Demigods, & Heroes is specifically for the Cleric, the class itself is a bit neglected in the early Little Brown Books. The Monastic Knights class treatment I made a while back was an attempt to fill that gap. Though with that class my intention was to revise the cleric from less of a priest and more into a holy warrior. A niche between clerics and paladins. A less restrictive paladin if you will.
I was having this conversation with David Pulver about the cleric and he mentioned he had written an entire treatment for the Cleric for his homebrew campaign. I was immediately intrigued and we soon agreed to publish it as an OD&D supplement. That was two years ago.
This project has been sitting on my backburner that long. I'd pull it out dabble with it, work on layout, and sketch some art, then put it back. I'm sure David was wondering if it would ever get made. Well, that time has come. It is a book that fleshes out the cleric class without breaking game balance. We'd joked that we could call it Old School Religion, OSR if you will, but ultimately Saints, Gods, & Relics was a more fitting title.
The book is now available in PDF & Print, and as a bundle.
https://www.drivethrurpg.com/product/401377/Saints-Gods-and-Relics