Zealotry and Spain's UA are both kind of useless right now, because

faith purchasing units is too expensive. The individual faith costs of all units are individually set for each unit in the game, and roughly follow this formula:
100

starting in ancient
+100

for each step at Classical, Medieval, and Renaissance, Modern
+200

for each step at Industrial and Atomic
No additional cost scaling at Information
+50

if it's a boat
plus an extra +250

for missile cruisers, nuclear subs, and mechs
proposal: Halve all scaling, and remove the extra bumps at Industrial and Atomic, add one for information, remove the boat tax.
100

in ancient
+50

for every era after that, including information
no extra cost if it's a boat
Zealotry is an entire belief slot. 1/5 of your religion. lower amount of

vs

and the no-XP loss for purchases doesn't make up for the fact that

is just more valuable and harder to generate than

, and that significant opportunity costs were incurred to unlock buying military units this way.
Spain is currently at the very bottom of the civ balance, and at least 1 significant part of their problem is that their ability to

-purchase boats is both more situational and more expensive than zealotry's land units.