A great deal of fundamental ideas which we all accept as a matter of social justice came from Christians that was not accepted by, say, the leaders of Roman society at the time. Now, the history isn't perfect, and there is plenty you can use against it in the Bible and in Christian history. But what does evolution give you? Evolution says that certain critters have certain advantages over certain other critters, and that using those advantages allows the critters with those advantages to pass on their genes. If materialistic atheism is true, there is nothing in reality that supports treating people with prima facie disadvantages as equal, as opposed to simply using your advantage over them for you and those you feel close to.
It is not as if everything in morality is religious, it's not. There are two forces in human nature that very often push us in a moral direction: social utility and sympathy. But we get social utility from those like ourselves (the social disutility of being a n-lover at a Klan meeting should be obvious), and we tend to lack sympathy for others when the others are one of "them" and not one of us. The antidote to this is found in the parable of the Good Samaritan but it cuts against human nature to a very large extent.
I find it distressing that movements within atheism are starting to see the conflict between belief and unbelief as a war, not a debate, and are starting to adopt an us vs. them mentality. I remember the well-intentioned ideas that launched the French Revolution and the Russian revolution, and remember also where these movements ended: with guillotines and gulags. As a result I am concerned about what is going to happen if the secularist movement today gets a lot of political power. These people started off with combining secularism with a passion for social justice, and look what happened to them. The death tolls of the Soviet Union far outstrip all the "holy horrors" of Christianity, such as the Inquisition. It's not even close. One commentator over on Debunking Christianity once started talking about putting Christians in camps and preventing them from reproducing. Of course most people of that mindset don't actually come out and say this stuff, but that doesn't mean they would resist the temptation if they thought they could actually cure the great "mind virus" that way.