Question

To sin by silence when they should protest makes cowards of men. meaning

Answer 1

Basically, staying quiet or not saying anything (To sin by silence) instead of standing up for what you believe in (When they should protest) therefore states that people are fearful or gutless (makes cowards of men). I believe that he is stating that if he remained silent about his beliefs, like slavery, then that would mean that he is a coward and since he is a leader and therefore a representation of men that will cause them to be cowards also. Lincoln is no coward and because he did not remain silent he changed the world into a much better place.