Holy Resistance isn’t a stat in the game, but level based resistance is in PvE - each mob/boss at higher level above you gains either +5 or +8 resistance to every spell school INCLUDING Holy which will result in PARTIAL resists to your Holy damage attacks - this cannot be overcome in any way even with Spell Penetration.
That being said, Smite is still a castable spell, which means that it still has to actually land first. Without enough +spell hit gear, your damage spells therefore have a chance to be fully resisted even in the absence of any Holy Resistance on the target.
Does that make sense?
TLDR - you will NEVER get a partial resist in PvP but you run the chance of getting one in PvE against raid mobs/bosses due to level resistance:
1067 SoC crit (178 partial resist)
Published on Nov 19, 2006