to drink the Kool-Aid
When someone has been persuaded to join a cause due to peer pressure. Meaning a persuasive personality has gotten you to believe in their cause. Usually has a negative connotation. This is a reference to the Jonestown mass suicide of 1978 when a cult leader mixed cyanide in Kool-Aid and had his followers drink it.
“Did you see Sue today?” “Yeah, she really drank the Kool-aid didn’t she?”