For those who didn’t get the title, check the famous C10k problem.
Designing a religion for one billion people.
Despite the Earth being still the same, it’s more important nowadays than ever for religions to handle hundred million of worshippers simultaneously. After all, the population growth rate is still positive.
The God is great, and omnipresent. So each (living) person should get sufficient share of it for its own purposes. Obviously, the God is not the bottleneck.
You will find some inspiration at the religions which hold great market share for centuries. It is certainly worth reading the Old Testament, which was later expanded with the New Testament. You may also find a rework of the same ideas the Quran which was published few political implementations later. If you are interested in low-level foundations, check some books from this list.
Your religion should be based on some moral rules. If you, for instance, draft such rules that praise only yourself, it won’t last long and soon you’ll find there’s too big competition among such systems. There is a complex field of study how to design such rules and since it is logically constructed it shouldn’t be a problem to grasp them for any person (provided they have the necessary theoretical background).
Fuse against erosion
Typically you deploy the religion via oral tradition or prophets. For it to last long enough to collect the one billion believers, you need to design fuses against erosion. Quite reliable is the declaration of the last prophet, beware that this method is applicable when you have final version of the religion ready. It’s difficult to introduce new patches afterwards. You may find the inquisition a more comfy way of control after deployment.
Create a manual (see Books). However, don’t be too explicit. Use allegories and don’t be afraid to make it poetry to further blur the meaning and broaden possible interpretations.
Take this idea and wrap it to liturgy and similar rituals. If you want to make it really bulletproof, promise social justice in the afterlife – no one can protest against it.
Avoid any single points of failure and be sure you make everything as distributed as possible (see, the omnipresent god is a useful idea). Making a single random place super-holy, doesn’t really scale well.