If the priests could generate experience, the need for TP would not be so essential.
There could be a card that makes priests cheaper and generates experience garrisoned in churches (if they think it would be broken to have these qualities by default).
The card might look like this:
Priests are 50% cheaper and generate 0.20 experience per second. + A free priest, or + a church cart.
Priests limited to 5 and do not occupy population. (This is by default)
Nearly outdated stuff that could be useful. (Potential improvements)