What should go into the one page description of your religion?
Most RPG religions have deities that are powerful beings with cosmic concerns. I deliberately chose the term powerful beings. As most deities have limits on their reach and knowledge. Sure they can smash a PC, or kingdom like a bug but in a nutshell they are thought of people who have a lot on their mind. In many ways they are like a parent to the mortal races.
This conception of deities gives us a good starting point for describing them. Think of your deity as a person with cosmic concerns. They had a beginning, they have a history. All of this shapes the personality they have today. You don't need to go into great detail, a sketch is all you need to start with.
From the deity's motivation you can derive a set of teaching. Come up with a half dozen or so tenets.
Set the deity within the history and cultures of your campaign. This is another area where you can go overboard so try to limit yourself to a paragraph. What this helps with is to decide the scope of the religion. There are a lot of ways you can go but the main choices are cult, local, cultural, or universal.
A cult religion has a handful of followers and are mostly secretive. Cults are traditionally associated with evil gods. On the flip side nearly all religions start out as a cult at some point.
A local religion is tied to a specific place. Gods of city-states also fall into this category. For example a toad god that protect a large swamp and all those who live it. Because of the turmoil of the region he also become a protector god of refugees who traditional hid in the swamp as the invaders pass.
A cultural religion is found throughout a particular culture. Two variant to consider are cultural religions that are religions of state and it opposite a cultural religion that followed by a conquered or oppressed populace.
A universal religion is found throughout multiple cultures in multiple region. The key attribute is that they teach something that has universal appeal. For example Dannu's teachings of living in harmony with the earth, home and hearth is far more likely to be followed by many cultures than the Toad God's obession with a swamp.
Universal religions can rise by a variety of means. A varient where a culture become dominent over many regions and enough time passes where it becomes fused with the local cultures (like Rome). Another variant is continued contact with longer lived races like the elves. The continuity offered by the elves has a powerful effect on the surrounding human cultures resulting a shared core set of belief. In someways this parallels the effect of China on the surrounding regions.
This is not commonly done but I found that having some type of calender greatly add to the players sense of place. Now this does not mean I hand the player a calender of dates. But rather periodically a festival or a special day occurs. Dates to consider are Traditonal founding day, Important dates of the deity life, seasonal festivals.
Up to this point we have defined little that will have an impact on adventure prep. With the belief defined, a basic mythology, the background, and calender you should have enough information to setup what type of priest that follows the deity.
I find it useful not to think of the priesthood as a organization of the cleric class. But rather than a description of all those who make serving the deity their primary profession. So for the forest god he may have a order of cleric who job is to maintain sanctuaries, a order of fighter who protect the sanctuaries, and a order of rangers who patrol the woods between and carry messages between the sanctuaries.
The result should be one or more template you can use to flesh locales four your adventures. Think of the guidelines for orc tribes, and merchant caravan in 1st edition's Monster Manual.
The body of the deity's believer is more than the priesthood. So having a paragraph describing what typical adherents are like will help in prepare adventures. Perhaps they live under some strict code, are they open to talking with outsiders, do they try to convert, and so on. The end result is another template you can apply to help you when you need to create a village of followers of Mitra vs a village of followers of Set.
Put some thought in how temples and other important religous structures are laid out. There will be variation based on locale and regions but in general structures for religous purpose follow a pattern. Like the cross layout of many of the great cathedrals.
Finally if you have note any neat ideas you have for your religion that don't fit anywhere else. And if you need combat stats here is where they would go.