To unlock the Paladin's Secret trophy in Far Cry New Dawn, you will first need to gain access to Expeditions in Prosperity. Once you have those available, you will need to play the first four Expeditions once so that the 5th Expedition called Government Plane Wreck becomes available.
The spawn point for the Expeditions is random. Once you begin, you will need to look for the large and rusty looking plane wreck. Enter the plane and take out any enemies you might come across. Once the area is is clear, interact with the laptop on the desk found on the bottom floor by holding the Square button.
This will activate the Splinter Cell easter Egg. Head up to the next floor using the stairs at the back of the plane, and you will notice that a door a the back of the room will open up to reveal Sam Fisher's outfit. Take the outfit by holding the Square button, and you'll unlock the Paladin's Secret trophy.