Lower level priests and blood sacrifices.

[Morkilus]

Page 93:

Blood Sacrifices To conduct a blood sacrifice, a priest of the appropriate nation can sacrifice a number of blood slaves up to and including to his priest level. The province he occupies must contain a temple. For each blood slave thus sacrificed, one temple check (described above) is generated. Thus, blood sacrifices can be a powerful method of spreading dominion.

I can't say anything about Mictlan's priests, since they're not specifically tabulated into the manual (yay, something for the Wiki, finally!)