Since some people have the inability to read long sentences, I've shortened the solution. Walk around the apartment, examine things. When Nathan shows up, do nothing.
Trophy.
If interested, the ORIGINAL (but be careful, there are words) solution is here.
*** Spoiler - click to reveal ***At some point early in the game, when playing as Jayden (the FBI Agent), you'll head to the house of a suspect named Nathaniel. You'll learn he's a bit of a religious individual, examine the house, for clues.
Check the various pieces in the main room (walls and a table), go to his room, check out his bed, nightstand and dresser. His bathroom and his kitchen. Head back into the main room and Nathaniel will return. After a few questions, he'll accuse Blake of being the Anti-Christ and point a gun at him.
Jayden will pull out his firearm, do not shoot Nathaniel and you'll earn this trophy. Shooting him will earn you the 'Blunder' trophy. Blake will never be killed by Nathaniel.