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.
First rule of politics: learn to count.
Socialism needs power, not purity. Power comes from the size of the support base.
Socialism has historically been helped by religious support same way as it's been helped by nationalist support.
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.
MLK jr was a minister