Im building a cleric and I am confused about clerics and shields. I did have another question but found it answered here. My remaining question is when a cleric casts a spell while holding a light shield and provokes an attack of opportunity, do I still get the shield bonus to AC or is the act of casting the spell cause me to lose it? Thus for longer castings I lose out on the shield bonus?
If you use the hand holding the shield (only possible with a buckler) for your somatic components, you lose the shield bonus to AC for the rest of your turn. If you use your other hand, you keep it.
The buckler is the only shield that allows to wield a weapon or cast a spell with the hand that holds it :
Benefit: This small metal shield is worn strapped to your forearm. You can use a bow or crossbow without penalty while carrying it. You can also use your shield arm to wield a weapon (whether you are using an offhand weapon or using your off hand to help wield a two-handed weapon), but you take a –1 penalty on attack rolls while doing so. This penalty stacks with those that may apply for fighting with your off hand and for fighting with two weapons. In any case, if you use a weapon in your off hand, you lose the buckler’s Armor Class bonus until your next turn. You can cast a spell with somatic components using your shield arm, but you lose the buckler’s Armor Class bonus until your next turn. You can’t make a shield bash with a buckler.
With a light shield, you will need your other free hand cast your spells, and you will still be able to defend yourself against attacks with your shield without any penalties.