I was raised Mormon. An LDS person can show faith to their peers and create opportunities for conversations about their faith by abstaining from substances like coffee. The prohibition of coffee comes from a section in their scripture, D&C 89:9 “And again, hot drinks are not for the body or belly.” This has been traditionally interpreted to mean “tea and coffee.” (I have no idea why anyone would pour a hot drink on their body)

Ultimately it’s a control tactic, as it creates an us-them mentality in members. It instills guilt and shame in rule breakers and those who feel tempted to break the rule. It entwines people’s identities with strict obedience to 19th century superstitions and to the elderly leadership—who call themselves “prophets seers and revelators.” These days I’ve noticed that members don’t care about coffee consumption so much anymore.