Why can't I convert Muslim provinces?

I am playing as Castille and have captured Granada very early on as part of the Reconquista national mission.

However, this leaves me with three provinces of Sunni religion in my otherwise Catholic nation. From the beginning I was unable to convert them, because it said it would take indefinitely due to the following modifiers:

• Base value: +2%
• Sunni: -2%
• Earth Rampart: +0.1%
• Provincial Tax Income: -2.5%
• Local Missionary Strength: -2%

I have continued to play for many years now, but I am still unable to change these modifiers. This leaves me with a hotspot for rebels and no way to reach religious unity.

1) What do Provincial Tax Income and Local Missionary Strength mean and how can I change these modifiers?

2) What is the best way to convert Muslim regions to Catholicism?

1a) The wealthier a province is, the more difficult it is to convert. Look at a correlation between the economic strength of a province, and that modifier. 1b) The "Local Missionary Strength" modifier is a natural feature of Sunni Islam, it reduces the effectiveness of your missionaries there by 2.5%, the stock strength of a missionary varies. You can see you bonuses and maluses on the religion tab.

You cannot remove the Local Missionary strength modifier from the Sunni faith outside of creating a mod for your game, so the only way you are going to even have a chance of converting Islamic provinces is with the Religious idea's "Divine Supremacy".

2) I would say the best way to convert Islamic provinces is to pour points into the Religious idea, and then starve them for wealth by supporting rebellions (which sap province wealth over time).

While the religious idea is good for this, it also let's you have one less idea tree to spend on, maybe, better ideas. Many Catholic countries have national decisions which can be unlocked by simple research that give you missionary strength of up to 4% (if I recall correctly) and you can always hire an advisor which gives you 2 extra %.