Exactly what the title says, Christianity in all its various strains still stems from the horrendous old testament, and with all the awful shit that endorses, as well as with things like Calvinism and the general doomsday cult foundations in the religion, why do some people still act so charitably towards it? Christianity has proven itself to be a morally corrupt and oppressive institution at every turn. Or maybe I'm just too negative? 
EDIT Ok guys I get it this was rather reductionist, and I definitely was disregarding all the good that religious folks have done, and that religion has inspired people to do, but again over correcting into enthusiastic support for religion can also be problematic.
I'm curious, which socialist causes would you say have been helped in a major way by religion? I know liberation theology exists but that's about the extent of my knowledge, and my baseline knowledge says that of the current and former AES projects, while there might be some religious base supporting the cause in latin america, I am unfamiliar with the specifics.
On a separate note, I would argue that the relationship between religion and support for left-wing causes is unlikely to be the same in the imperial core as in formerly colonized countries. As you say, the creation of a nationalist, anti-colonial movement has been useful historically in countries like China, Vietnam, and Cuba, but obviously we wouldn't expect an American communist movement to seriously attempt to rally around the flag. Similarly, I wouldn't expect a revolution in the US to attempt to ally with the domestic bourgeoisie as they did in China.
Well one example would be that many Quakers have been active radicals for a variety of causes over hundreds of years, including but not limited to; opressive Christian church structures, monastic heirarchies, slavery abolition, pacifism, democratic representation for all manner of people (women, minorities, non-landowners) over the years, factory workers rights, safe housing, fairer wages, prison reform, anti-poorhouse campaigning, environmental movements, LGBTQ+ rights, rights for refugees, and more.
Even just personally over the last two decades (I'm not a Quaker, but have Quaker comrades) I've engaged direct action with a disproportionate amount of Quakers against US imperialist bases, against airport expansion, refugee defence and de-arrests, the campaign for nuclear disarmament in the 90s and the anti-war movement in the 2000s, as well as pro-Palestinian activism.
And this month UK pigs raided a Quaker meeting house - a place of worship equivalent to them raiding a church - smashing down the doors and arresting people for merely providing a place for people to discuss a potential protest. The first time the state has done so in hundreds of years.
There's lots of socialist Quakers, some communist Quakers, and far far more that are still fighting for radical causes. And that's just one small branch of Christianity.
MLK jr was a minister