Oh, this is not possible. The game does not recognizes names of factions in this file, only the religions.


So, if you change all the religions of all the factions, you can to do this.
I mean, if all the other factions have the Islam or Catholicism and your faction is the only one with Orthodox Christian religion in the game, you can make the units to be Orthodox-only recruitable units by adding the religions { orthodox } at the end.

So, for example:


unit Mercenary Frankish Knights exp 2 cost 1080 replenish 0.02 - 0.10 max 2 initial 0 religions { orthodox }
unit Mercenary Crossbowmen exp 2 cost 860 replenish 0.08 - 0.25 max 4 initial 2 end_year 1300 religions { orthodox }