get to the heart of
Find or determine the most important or essential facts or meaning. For example, It's important to get to the heart of the matter before we make any decisions. The noun heart has been used in the sense of “a vital part” since the early 1500s.
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.