Greta Thunberg earned her moral authority by first refusing to speak.

Evidence

It is worth recalling that Thunberg’s first protest was her refusal, as a young girl, to speak. She learned about the ecological crisis, she saw how little was being done about it, and she stopped speaking to anyone beyond her family. She started speaking when things started changing—at first small things, like members of her family committing to becoming vegetarian, and then larger things, like millions of people joining climate strikes around the world. — Naomi Klein, Doppelganger, 498