Just adding a little bit to the roux here
Believe me, there's been a lot of debate about Christianity and its role in slavery and its aftermath.
I mentioned the black churches that split off from the Methodists and Baptists to have their own churches they led and felt comfortable attending b/c the white leadership either endorsed or excused slavery and/or excluded black pastors from leadership positions in the wider church hierarchies.
I mentioned the contrast between Sweden's national church vs the USA's motley assortment of locally-led churches that fractured along doctrinal and regional differences preventing much of a national religious consensus folks could accept or reject until the 1960's for both whites and blacks.
What this means is that secularism tends to flourish when the religious dogma has become less of a passionate issue. In America, we got close, to being as secular as Europeans but ever since the 1970's, there's been a backlash and now two generations later, we're drifting back to a more secular consensus.
The passionately religious have always been a minority, but they intimidate a lot of the casually religious to unreligious folks into treading carefully around them here.
How secure the secular population feels about telling the religious to step off depends on how secure they feel about things in general and by and large I'd say we're a rather anxious lot here in the States. We know the status quo is broken but don't dare try anything else b/c no other alternative really is acceptable.
Back to the OP, explicit socialism here in the States has been a non-starter since the 1920's. We'll try something packaged as a liberal social welfare program as a line item issue but not as part of a broader social movement.
I'm with you about the Harlem Renaissance figures, Langston Hughes and Zora Neale Thurston et al. They were cool folks with neat ideas and dreams. However, their ideas and politics and lifestyles were so far outside what the average person's concerns were that their example didn't translate to much cultural or social impact.
We Americans, whatever color we are, collectively don't embrace intellectuals of any stripe here in the States. It sucks but there it is.
For African-Americans to be weaned from churches, you have to have a lot more secular organizations involved and involving them. Eleanor Roosevelt tried to make New Deal programs address black poverty, illiteracy, and other social ills as well as what devastated the general population.
That was social engineering that Dixiecrats weren't going to tolerate so it didn't even get beyond the talking stage. If it did, say Huey Long and other progressive Southerners include blacks as part of their war on poverty, then you could see a secularizing trend from the Depression on.
Ending segregation wasn't easy or simple. As long as African-Americans were hyphenated Americans, seen by whites as subhumans, enemy aliens, or unfortunates marooned on a hostile shore, it was difficult for African-Americans to trust or embrace something or somebody outside of their community. Of course, it was a social absurdity and a crime that blighted millions of lives for centuries. In many ways, the effects linger.