Nothing discloses real character like the use of power. It is easy for the weak to be gentle. Most people can bear adversity. But if you wish to know what a man really is, give him power. This is the supreme test. It is the glory of Lincoln that, having almost absolute power, he never abused it, except upon the side of mercy.

Robert Green Ingersoll (1833-1899) American lawyer, agnostic, orator
“Abraham Lincoln,” Lecture (1894)
    (Source)

Ingersoll used the final phrase here frequently about Lincoln, e.g., in The Liberty of Man, Woman, and Child, an 1877 lecture, he wrote: "Abraham Lincoln was, in my judgment, in many respects, the grandest man ever president of the United States. Upon his monument these words should be written: 'Here sleeps the only man in the history of the world, who, having been clothed with almost absolute power, never abused it, except on the side of mercy.'"

The phrase "But if you wish to know what a man really is, give him power" is often attributed, without citation, to Lincoln.