In Mahou Galaxy, faith and deities work a bit like this:
A minor deity(one with more than 10,000 but under 1,000,000 adherents) on a planet provides 1 level of ambient faith, while a major deity(One with 1,000,000 or more adherents) provides 2 levels. Being in a location considered sacred by adherents to the deity, such as a place of worship or a historically important site, adds another 1 faith level. This is considered a bonus to the "group" level of faith determined.
The faith level in an area is largely determined by the GM, but there are some guidelines that should be used to help with the GM's determination. Here are some sample persons or groups of regular believers that would generate a specific level of faith when nearby. The farther away that person or group, the less impactful their local faith level. Faith levels are also important to some elementals.
Level Single Person
+1 Believer
+2 Zealot
+3 Devout Priest
+4 High Priest
+5 Avatar
Group size: Faith Level Modifier
1-9 +0
10-49 +1
50-199 +2
200-999 +3
1000-4999 +4
5000+ +5
And the faith level is also modified by distance. A zone of faith remains at full strength up to a mile out, but then diminishes with further distance as follows:
Distance Modifier
Greater than 1 mile -1
Greater than 5 miles -2
Greater than 25 miles -3
Greater than 100 miles -4
Greater than 500 miles -5
Faith in this fashion cannot extend farther than the entire reach of a planet, though some game mechanics can use this at a relatively close distance to a planet, particularly Elementals aboard space ships.
In these terms, your average citizen who believes in a deity, may make the occasional plea to them, but otherwise goes about their life as normal is considered a "Believer". Someone who devotes a large amount of their time to service of that deity or spreading their belief, including warriors who fight in that deity's name, are considered "Zealots". A "Priest" is a character who has devoted their life completely to service of a deity, usually with ritualistic clothing and various holy symbols on hand. A "High Priest" is a priest who has performed such exemplary service in the name of their deity that they are considered a senior member of their order. An "Avatar" is a person who is granted direct power by the deity in question, and generally performs a role under direct orders from that deity. When figuring faith level, a group of stronger believers can increase the faith level as would make logical sense, by adding the group size's level modifier to the group's faith level. For example, a group of 50 zealots would have a default faith level of 2, plus the group size modifier of 2, for a total of 4 faith levels. If they were part of a major religion, that would increase the faith level by +2, to 6, and being on holy ground would add another +1 to bring that total faith level to 7. When figuring the faith level of an area, take the most faithful believer first, then total the group including all who are considered within two levels of faith.
The actual existence of a deity is immaterial to the mechanics of faith. Someone with enough skill can convince people of a nonexistent deity or in the deific powers of a nondeity. However, this sort of thing CAN lead to the creation of an actual deity one way or another. If faith exists at level 10 or higher in an area for at least a year, at the GM's discretion an appropriate elemental may form, or a target of worship(either living or not) may be granted a new form appropriate to the worship they receive. This almost always takes the form of an elemental subrace. A newly manifested deity is usually a level 1 character, with mannerisms and personality traits derived from the specific kinds of worship and beliefs about that new deity.
In MSF High, however, it's a bit more simplified. Every day, the first time faith needs to be checked by the GM, they roll 1d4 and subtract 1. The result is the default faith for an area and a specific aspect or deity. Priests and other high-faith PCs and NPCs can obviously change that, and if it happens to be a holiday based on the deity in question, that default faith roll is instead 1d4+1.