The quote comes from a video by Hip-Hop radio show host Jay Smooth, which was quoted by Fred Clark and in turn by Steve Thorngate, both of whom noted the similarities with the thought of Reinhold Niebuhr. The video can be watched below (the quote comes from around the 2 minute mark). Here is a bit of the larger context of the quote to whet your appetite.
That mindset right there is what does as much as anything to perpetuate injustice all over our society. That assumption that only a “cretin” or a monster or a bad person would ever be racist or sexist or harbor any sort of bias or prejudice.
That right there is the Big Lie. There is nothing that does more to perpetuate injustice than good people who assume that injustice is caused by bad people. That’s just not how being good works. And that’s not how being a human being works.
The truth … is that all of us, as good people, are still naturally prone to doing bad things. We all have natural tendencies toward implicit bias and prejudice and bad habits. …
[I]t took me a long time to learn — and I’m still learning, every day — that if I just believe that I’m a just person, and therefore my choices are just, I’m going to be part of the problem.
There is a beautiful statement about the need to work at goodness as a craft, much the same way one works at one’s profession.