The priest holds sway over the congregation more so than any other individual member and so objectively is the most powerful element of the three. Atleast in the short term. 

The book being the longest living member is arguably more powerful in respect to all of time. 

The congregation is the expression of his power and the book is the loose contract between him and the congregation. 

How he is allowed to guide that expression of power. 

Just like a countrie's constitution and it's leaders.