Some monsters can have over 100% total resistance
So if Conviction subtracts -150% and the monster has 150% resist cold its resist will drop by 20% of the Conviction modifier which is -30% ending up and 120%. It will still be Cold Immune
A less technical answer is high level Conviction will usually drop monster resists to about 80% if they were Immune but not always :)
Not sure if it stacks with Lower Resist and Cold mastery.
So if Conviction subtracts -150% and the monster has 150% resist cold its resist will drop by 20% of the Conviction modifier which is -30% ending up and 120%. It will still be Cold Immune
A less technical answer is high level Conviction will usually drop monster resists to about 80% if they were Immune but not always :)
Not sure if it stacks with Lower Resist and Cold mastery.