The War of the Chosen expansion introduces three new character classes: Reaper, Skirmisher and Templar. They're not available first and have to be unlocked - after a few days, the player is enabled to play the Lost and Abandoned mission, which will grant them access to the first two. As concerns the Templar, this are just as simple.
After a few weeks in the game, the Covert Actions screen (the details concerning this mechanic are explained in the " Resistance's Covert Actions) will show you a new available mission for Templars - Locate Faction. Just send their two soldiers (one has to be at least a Sargent) and the mission will be concluded after a specified period of time, providing you with access to the new class and a new faction of the Resistance - the Templars. You will be able to choose from among more Covert Actions, as well as to neutralize actions of the third chosen one.
You might want to unlock the Templar as soon as you can. This will give you access to a powerful soldier fighting in close combat and will ensure the Templar develops well - they're all about melee combat, so it's a good idea to develop them a few levels ahead before really powerful enemies start occurring in battles.