During my short tenure as a game master, I have purchased and read a large number of campaign settings and have seen the numerous different ways the designers of these worlds handle the topic of religion and the deities that watch over the world. However, one form of religion almost always seems to be absent in these fantasy settings: Monotheism.
The majority of campaign settings tend to be polytheistic in nature. There are numerous gods and goddesses roaming the multiverse and acting as patrons for those mortals who happen to fall into one of the domains they have power over. Now, I can see why most people would go the polytheistic route in a campaign setting over the monotheistic route. With numerous deities, the players have more options to choose from and the presence of multiple gods and goddesses better mirrors the legends and mythologies that so many of us draw inspiration from for our games.
With that in mind, most people would probably see a monotheistic religion in their fantasy world as "restrictive". However, I believe you can make monotheism work within a game world rather easily with the following pieces of advice and guidelines.
First, let's tackle the whole "restrictive" argument. While a monotheistic religion could be seen as restrictive because players only have one god to choose to worship, this doesn't mean the characters all worship this one god the same way. For example, there could be numerous sects throughout the land who worship the True God in different ways. You might have one sect who is very much tied to religious dogma and puts a lot of importance on ritual and church law, while another sect is more free-spirited and believes as long as you live your life in the name of the True God and follow his basic teachings, you will be saved. Just look at some modern day monotheistic religions like Christianity and Islam to see how different sects vary in how they worship the same god.
Also, while there might be only one deity, there could be numerous patron saints that have more specific focuses for the characters to venerate. These saints would take the place of some of the typical roles for gods in a more polytheistic society, like the saint of knowledge or the saint of soldiers. While all these characters still worship the main deity of the religion, they find themselves venerating the saints that watch over their interests and professions as well.
I would also suggest implementing one of the rules the Eberron setting used for religion. Basically, in the world of Eberron, clerics did not have to be within one-step of their deity's alignment. This allowed for LE clerics dedicated to the LG Church of the Silver Flame. By implementing this rule, you could have a few "heretic" sects who worship the main deity in ways others would see as evil or have bishops of the church who have become corrupted by power. This would also add more options and flavor for your PCs to take advantage of.
Another idea I would highly suggest implementing is to make the main god of this monotheistic religion distant. Have this deity never present itself in a overtly obvious way in the world. Have the signs of its presence being extremely symbolic and could be interpreted as just arcane magic or something else. If the god is distant, you can have characters question their faith in a god that seems to remain silent and have scholars argue this god might not even exist at all and the clerics are all actually just sorcerers who are deluding themselves. With this, you can make your world and the religion more interesting and create some interesting story lines and adventures in your games.
Finally, just because there is only one god does not mean he only has one form. While a few lands might worship this god in a form that resembles Catholicism in the Middle Ages, these distant desert lands might worship the very same deity in a form that is closer to Islam, and the nomadic barbarians of the northern wilds might worship the god as the Earth Mother and the saints as her many children. While they are worshiping the same god, they just have different names and forms for him.
I hope this helps those who could never figure out a good way to implement a monotheistic religion into their games decide to give it a second try. While I like the polytheistic ways of doing things, sometimes you need a change of pace and a monotheistic religion might be the perfect thing for the job.