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Dr. Martin Luther King addresses civil rights marchers in Selma, Alabama, in April 1965. Keystone/Getty Images

Fifty years ago on this day, Martin Luther King, Jr. was shot and killed at a motel in Tennessee by fugitive James Earl Ray. King was born Jan. 15, 1929, and was assassinated April 4, 1968. He remains an iconic figure whose words of inspiration continue to ring in the hearts of people across the country and the world.

Below are 18 quotes from the civil rights leader to honor him.

1. "Freedom is never voluntarily given by the oppressor; it must be demanded by the oppressed."

2. "Injustice anywhere is a threat to justice everywhere."

3. "The ultimate measure of a man is not where he stands in moments of comfort and convenience, but where he stands at times of challenge and controversy."

4. "If a man hasn't discovered something he will die for, he isn't fit to live."

5. "Life's most persistent and urgent question is, 'What are you doing for others?"

6. "Everybody can be great ... because anybody can serve. You don't have to have a college degree to serve. You don't have to make your subject and verb agree to serve. You only need a heart full of grace. A soul generated by love."

7. "Our lives begin to end the day we become silent about things that matter."

8. "I have decided to stick with love. Hate is too great a burden to bear."

9. "Forgiveness is not an occasional act; it is a constant attitude."

10. "Mankind must put and end to war or war will put an end to mankind."

11. "What is wrong in the world today is that the nations of the world are engaged in a bitter, colossal contest for supremacy."

12. "I have a dream that one day this nation will rise up and live out the true meaning of its creed - we hold these truths to be self-evident: that all men are created equal."

13. “Now there is a final reason I think that Jesus says, ‘Love your enemies.’ It is this: that love has within it a redemptive power. And there is a power there that eventually transforms individuals.”

14. “Hatred paralyzes life; love releases it. Hatred confuses life; love harmonizes it. Hatred darkens life; love illuminates it.”

15. “You know my friends, there comes a time when people get tired of being trampled by the iron feet of oppression. There comes a time my friends, when people get tired of being plunged across the abyss of humiliation, where they experience the bleakness of nagging despair. There comes a time when people get tired of being pushed out of the glittering sunlight of life's July and left standing amid the piercing chill of an alpine November. There comes a time.”

16. “We cannot walk alone. And as we walk, we must make the pledge that we shall always march ahead. We cannot turn back.”

17. Darkness cannot drive out darkness: only light can do that. Hate cannot drive out hate: only love can do that.”

18. "We must accept finite disappointment, but never lose infinite hope."