Longinus is basically the weapons dealer for the Golden Path and a religious man. His missions will become unlocked after finishing Sermon on the Mount and are generally short missions with the same overall structure. You basically have to follow someone to their hideout, steal their diamonds and then bring them to a drop-off point.
It definitely helps to have a gyrocopter for these missions, as they will help if you want to remain undetected, although it doesn’t matter if you do or not. Head to the first waypoint and wait until you see the guy you need to follow drives past. Keep following him, whether you’re detected or not, until he reaches the vault.
Once you’re there, kill the smuggler, as well as the other enemies, then loot the smuggler’s body to get a key. Enter the vault and look for a table with some candles, as that’s where the diamonds will be. Grab them and exit the vault, only to be greeted by some enemies. Kill them all, then go to the waypoint to drop the diamonds off and finish the mission.