This quotation was frequently used by President Kennedy to make the point that on important moral issue it is wrong to sit on the fence and refuse to take a side. Martin Luther king, Jr. used a variation of the quote (“The hottest place in Hell is reserved for those who remain neutral in times of great moral conflict.”). He added, “He who passively accepts evil is as much involved in it as he who helps to perpetrate it. He who accepts evil without protesting against it is really cooperating with it.”
According to the John F. Kennedy library website: “One of President Kennedy’s favorite quotations was based upon an interpretation of Dante’s Inferno. As Robert Kennedy explained in 1964, “President Kennedy’s favorite quote was really from Dante, ‘The hottest places in Hell are reserved for those who in time of moral crisis preserve their neutrality.'” This supposed quotation is not actually in Dante’s work, but is based upon a similar one. In the Inferno, Dante and his guide Virgil, on their way to Hell, pass by a group of dead souls outside the entrance to Hell. These individuals, when alive, remained neutral at a time of great moral decision. Virgil explains to Dante that these souls cannot enter either Heaven or Hell because they did not choose one side or another. They are therefore worse than the greatest sinners in Hell because they are repugnant to both God and Satan alike, and have been left to mourn their fate as insignificant beings neither hailed nor cursed in life or death, endlessly travailing below Heaven but outside of Hell. http://www.jfklibrary.org/Research/Ready-Reference/JFK-Miscellaneous-Information/Dante.aspx