Go in the data folder in M2TW's directory,and find the file descr_campaign_db.xml.Right click it and press edit.Scroll down until you find these lines:
Code:
<religion>
<max_witches_per_region uint="1"/>
<max_witches uint="20"/>
<max_heretics_per_region uint="1"/>
<max_heretics uint="80"/>
<max_heretics_conversion_modifier float="1.0"/>
<max_inquisitors_per_region uint="1"/>
<max_inquisitors uint="40"/>
<heretic_conversion_rate_modifier float="0.0075"/>
<heretic_conversion_rate_offset float="0.025"/>
<witch_conversion_rate_modifier float="0.0"/>
<witch_conversion_rate_offset float="0.02"/>
<inquisitor_conversion_rate_modifier float="0.010"/>
<inquisitor_conversion_rate_offset float="0.04"/>
<priest_conversion_rate_modifier float="0.004"/>
<priest_conversion_rate_offset float="0.015"/>
<witch_creation_modifier float="1.0"/>
<heretic_creation_modifier float="0.4"/>
<inquisitor_creation_modifier float="0.3"/>
<min_cardinal_piety uint="5"/>
<convert_to_heretic_base_modifier float="10.0"/>
<convert_to_heretic_unorthodox_modifier float="0.7"/>
<inquisitor_turn_start uint="20"/>
<inquisitor_target_crusades bool="false"/>
</religion>
See the line I highlighted in red.It determines the piety value that a priest needs to have in order to be a cardinal.All you need to do is to change the 5 to a number with a higher value.It must not be more than 10.It may cause problem to the game.