Check out this thread:
https://forums.totalwar.org/vb/showthread.php?t=55030
And read the BI manual, it explains it pretty well...
For the specific question... The conversion rate deterine which direction your settlement is going and how fast. Even if you have inflences from all three religions only the higest influence counts (at least according to the manual)... So if you have +10% Pagan, +25% Christian and +10% Zor(whatever) then you end up with your present religion balance being converted to christianity at a rate of %5 (I think, the manual was not that clear)...
The rate of converstion is governed/influnced by the religion of surrounding provinces, the religion of characters (including diplomats and spies if the have the appropriate traits) in the province (not necessarily just in the settlement) and by any religious structures you build.
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