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If the target province has a religious building, that will counter the effect of your preachers.

One of my abandoned Turk campaigns, I spammed the Mediterranean all around with âlims. IIRC, Sicily and Malta were sent the preachers (both val. 0) at the same turn. After a while, one of them was 70-some percent Muslim while the other was stuck in 30s and had a church on it.
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