Read the Hallowed Scroll and speak to Dark Cleric Duesten in the church in Deathknell.
Ah, while you were off dealing with the mindless Scourge, this scroll arrived for you. I would think it's some matter of importance as it seems it bears the seal of the priest trainer Duesten. I would take some time to read it before heading out again.
Ah, so between fire, plague, rioting, and much worse, one more of the "faithful" comes to my door. Well, let me bid you welcome, <class>. And allow me a moment to explain a thing or two before you decide it's time for you to seek whatever drives you forward: the Holy Light no longer concerns you, the spirits of your forefathers are fairy tales, and creatures from the Nether don't want you.
Do you understand me so far?
There is only one thing you must know: we have survived through will alone. It is faith in ourselves that separates us from others, and with our powers, we will cause great change in all of Azeroth. The weak will come to lean on you. The lepers will call you Lord. And the ignorant will look to you for guidance. It is my duty to make sure you have the necessary tools so when the time comes, you are prepared. As you grow in experience, seek me out. I will teach you greater powers if you are ready.