Religious Zealots says that they up the mortal threshold of the casting priest if the R.Z. is within 3xCommand Range of the casting priest. Furthermore, they do not add this bonus if they're channeling.
Does a Priest who is also a religious zealot gain the +1 M.T. bonus when it's casting a spell? Since it's (obviously) within its own command range, and isn't aiding in channeling either.