First, you have to know, that you get Faith in two waves during a sermon.

The first is the base amount. This you get every time, and it is not effected by the selected prayer.

The secound wave is the prayer bonus. This depends on the prayer

Keep in mind, that you only get the faith from the prayer bonus, if the prayer is successfull.

This amount is not effected by the prayer, it only depends on the church rating.
For every rating point, the chance for 1 faith point is increased by 20%.
This means, for every 5 church rating, you are garanteed to get 1 faith

First, most Prayers have a %-modifier* in their effect.
This one is applyed first, so the first part of your bonus-faith is a percentage of your base faith.
The secound part are just additional faith Points. At the end of its effect text, each prayer has this line:
Faith (xN), ...

N just tells you, how much you get as secound Part.

(*) it seems like the game is always rounding down.

Here just a quick example.

Let's say, your church rating is 21.
This can be achieved for example, by building all basic benches, two basic confessionals, and two basic candelabras.

Lets compare two prayers:
- The one you get for opening the church:
Casual prayer
Effect:
Faith(x2)

- The first one you are able to make:
Prayer for faith (copper quality)
Effect:
Sermon money: +20%
Sermon faith: +50%
Faith(x1), Silver Coin1

With a church rating of 21, you have 4 faith garanteed, and a 20% chance to get a 5th one.
Let's assume, you're unlucky, so the faith from the first wave is 4.

Both prayers need at least 10 church rating for 100% chance to work, so you are guranteed to get the faith from the secound wave too.

The casual prayer has no %-bonus, but has a (x2), so you get a fixed 2 additional faith, which makes a total of 4+2=6.

the prayer for faith has a 50% bonus, so, at first, you get 4*0.50=2 additional faith. Also, it has a (x1), so one additional fixed bonus faith. This makes a total of 4+2+1=7 faith.