I don't have a problem with actual Liberals being religious and actually if I had to guess just at looking at the Democratic Party, Liberals tend to be religious. But as a Liberal myself, I have a problem with being both religious, or an Atheist, because I simply don't know if there's a God, or not. Liberalism, is based of reason and evidence, not faith. And if you take the position that God doesn't exist, but you can't prove it, because no one actually knows, if we were all real about this, you have faith even as an Atheist that God doesn't exist.
I just don't work that way as a non-religious person who generally doesn't go by faith. I trust people, sure, because they've given me reason to trust them. But again that is based on actual evidence. Not having some grand vision and beliefs that there is some higher power out there watching over everyone. There isn't some Liberal God (at least that I know of) giving Liberals all of their powers and ideas. We developed them based on knowledge and evidence from what works. Education, freedom, responsibility and let people make their own decisions.
And because I don't have faith in whether there's a God or not, I'm neutral on the subject. I'm an Agnostic and I take strong positions on issues where there's clear evidence one way or the other. Which is how I have my liberal principles that are built around individuality, choice and responsibility. Because that is where the evidence suggests that I should be. Put out all of the facts and information about all of the issues on the table. (Just don't eat them) Educate everyone as best as possible, but at the end of the day let people make their own personal and economic decisions and hold them responsible for them.
I don't hold these values, because I have faith that they will work, but because there's clear evidence that they will work and have worked. Religion, is the opposite of that at least as it relates to God. "We can't see God personally, but we know he's there looking after us, because we have in faith in them." That doesn't mean anything to me and I'm not impressed by that. But others of course can make their own decisions. Which is what I believe in as a Liberal. It is one thing to have strong moral religious values about how you look at life. It is another to say, "this is what is right, because God told us that." How would you know that, did you ask him? Did you see him write that down somewhere? How do you know that God is a man? I don't have to answer these questions, because I don't believe in God, because I don't know if one, or many exist, or not.