HOW do I cast Priest Spells?

[tecknode]

First, how does a Cleric cast memorized spells in the "Priest Scroll" normally?

My Cleric can't.

Now my current companion is actually a Cleric-Thief so is the ability to cast a Priest Spell hampered by that?

If yes, why since he CAN cast spells in the Mage Book? This would be inconsistent.

Is there a fix?

[Armanz]

You're likely confusing Mage and Priest Book here. Clerics and Druids use the Priest Book, while Mages, Sorcerers & Bards use the Mage Book.
Mage spells are disabled by wearing armor but Cleric spells aren't.
You use the icon with the moon and stars just like with Mages to cast :)

[Finrod]

Priest spell book and mage spell book are bound to different key bind