The exalted leader (what priest means) are in fact, just the chosen ones. When Jesus (a) left, Elijah (a) took the position of light and holy spirit, but public social and political leadership was taken by Simon (a) and Elijah (a) was hidden. Simon (a) is not chosen exalted leader from God, and so his successors are not infallible in interpretation of the gospels nor are the leaders guaranteed to remain pious since God is not choosing them but people are choosing them, and can be wrong.
The Church although sanctioned by God till return of the chosen exalted leaders, which was Mohammad (s) and his family (a), are not guarantee from God to lead towards guidance.
Today Islamic social and political leadership is in the scholars and God has sanctioned believers to rule themselves by most knowledgeable and trusted in wisdom among them, but their choice is not a guarantee of guidance.
There is a vast difference between a chosen exalted leader appointed by God and those appointed by people.
Guidance on earth and true interpretation of the book is through the chosen guides and leaders. Why scholars can help facilitate understanding, they are not to replace the thirst we have to for the words of Ahlulbayt (a).
The believers spiritual guidance has to rely on the rope of God. I agree a book of God is not sufficient without guidance of the leaders it designates, but, this doesn't mean we choose leaders and their interpretation is a guarantee. Rather there is a chosen king and leader among us we have to turn to and get guidance from.
