root of the matter
The essential part or cause of something, as in We still don't understand what happened; we must get to the root of the matter. This expression was first recorded in the Bible (Job 19:28).
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.