I believe that is where his strength came from:
"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. Just keep being friendly to that person. Just keep loving them, and they can’t stand it too long. Oh, they react in many ways in the beginning. They react with guilt feelings, and sometimes they’ll hate you a little more at that transition period, but just keep loving them. And by the power of your LOVE they will break down under the load. That’s LOVE, you see. It is redemptive, and this is why Jesus says LOVE. There’s something about LOVE that builds up and is creative. There is something about hate that tears down and is destructive. So LOVE your enemies.”
- Martin Luther King Jr.