On day 12 you will be asked by the reverend to take down a poster and bring it back to him. On the next day your sarge will tell you about a cult and you will need to take down all the posters and get rid of all the cultists. Once done go to the church where you will meet a little girl. After speaking with the girl go to find her father & uncle. Once found follow them and wait till they enter the apartment. Press any buzzer and get inside then select Wait twice and one of the followers will shoot the cultist priest then achievement will pop.