Earlier quoted context omitted.
You've confused me. Are you saying that "leftists" cannot quote king because he was Christian? Why's that matter? Also, quotes can be used for many purposes outside of their origin, e.g., love thy neighbour, etc.
No, I'm saying that I know actual leftists (not sure what your quotes are for) who will only post King quotes when the topic is "White liberals are unhelpful" and "anti-capitalism", typically only posting quotes which don't include religious terms. Then, they will unironically claim that his legacy is being erased by the folks who only ever quote him from "I have a dream". To which I say: yes - his legacy is being er…
It's always left me a bit uneasy when people speak of "legacy being erased" as it's often untrue. In this way, I disagree with both yourself and those individuals you quoted. For me, leaving out details is commonplace in all aspects of life. That doesn't entail erasure. I doubt that people consider the complete background of authors when sharing inspirational quotes. The "I had a dream" speech and quote is powerful and meaningful to the individual because of what it reads, not necessarily who wrote it. That's why you'll see people quote people who are arguably did good ... but also a lot of bad, e.g., Winston Churchill.