Here is the full list of all 20 Forgive Me Father achievements. It takes around 10-12 hours to unlock all of the achievements on Windows.
Complete the game with the Priest
Unlock all the secrets in the Rooftops level
Finish any level without taking damage
Finish any level with 1 hp left
Shoot a bullseye with your gun on Wetlands
Survive 20 waves in Endless mode
Survive 10 waves in Endless mode
Stun 4 enemies with one use of Priest's Stun skill
Use Godmode to save yourself when your health is lower than 10
You found and used the secret gun
Finish Forest level without killing any enemy
Finish any level on 100%
Cleanse the Streets from all of the balloons
Complete the game with the Journalist
Glitch has been defeated
Big Ugly Fish has been defeated
Azyzz has been defeated
Kill 5 enemies during a single use of Voodoo doll skill
Ca'tharsis has been defeated
Kill 10 enemies with single herbs skill use