take into one's confidence
Trust someone with a secret, as in She took me into her confidence and admitted that she was quitting next month. This idiom uses confidence in the sense of “trust,” a usage dating from the late 1500s.
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.