drink the Kool-Aid

Etymology
Referring to the Peoples Temple cult movement, whose members committed mass suicide in 1978 by drinking a powdered drink mix (popularly, but erroneously, believed to be Kool-Aid) laced with cyanide.

Verb

 * 1)  To come under the influence of a leader, organization, or movement to the point of believing uncritically in an espoused idea or ideology.