“In the end, we will remember not the words of our enemies, but the silence of our friends." - Martin Luther King Jr.
"To sin by silence when they should protest makes cowards of men." - Abraham Lincoln
"There may be times when we are powerless to prevent injustice, but there must never be a time when we fail to protest." - Elie Wiesel
“Each time a man stands up for an ideal, or acts to improve the lot of others, or strikes out against injustice, he sends forth a tiny ripple of hope, and crossing each other from a million different centers of energy and daring, those ripples build a current which can sweep down the mightiest walls of oppression and resistance." - Robert F. Kennedy
Generally: For almost every piece of advice there are useful contrasts. More than useful, actually: necessary. Ideas that seem reasonable (even wise) in isolation often don’t ‘survive contact’ with broader thinking. In other words, an isolated idea may not prove applicable for a particular context.
Here are more quotes that can help weigh your choice to speak:
"Tact is the art of making a point without making an enemy." - Howard W. Newton (not Isaac Newton)
“What you do speaks so loudly that I cannot hear what you say." - Ralph Waldo Emerson
Specifically: The early and outspoken critics sometimes get their heads cut off. Sometimes this galvanizes change. Sometimes this just helps the decapitator find his enemies more efficiently. Sometimes it is wiser — and more ethical — to quietly (even secretly) organize and act when you have a critical mass.
“The nail that sticks out gets hammered down." - Japanese Proverb
“A single arrow is easily broken, but not ten in a bundle." - Japanese Proverb
― Desmond Tutu