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.
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.