# But I will tell you

"But first I will tell you" 

# what is written in the Book of Truth

"what the Book of Truth says" or "what someone wrote in the Book of Truth" 

# There is no one who shows himself to be strong with me against them, except Michael your prince

"The only one who shows himself to be strong with me against them is Michael your prince" or "Michael your prince is the only one to show himself strong with me against them"

# who shows himself to be strong with me against them

"who proves himself to be strong with me against them" or "who helps me against them"

# Michael your prince

"Michael, the prince of your people"