First off, I recognize that “Democratic Party” and “liberalism” are not one and the same. Nevertheless, in order to avoid the OP turning into a long meandering sprawl of words, I’m going to have to conflate the two a lot.
As far as I can tell (as a Christian-and-somewhat-right-leaning Doper), liberalism’s substance fully reflects what you would expect of atheism: The Universe and life was made naturally via Big Bang/evolution, there is no intelligent design, the Bible is historically and scientifically inaccurate, people should be free to do as they please if it doesn’t harm others (if you’re gay, great, if you’re trans, great, if you want to have safe consensual sex with a hundred different legal-age partners, great), abortion is fine, prayer is useless, there is no reason women should ever be on a lower rung than men, God never steps in to intervene, religion is the cause of a great deal of harm and death in the world, we answer to no supreme deity but only ourselves (fellow mankind and society), miracles are impossible, there is no life after death, and everything has a scientific or natural explanation.
But that is not liberal practice. You never hear a Democratic presidential candidate openly say, “There is no God.” The Democratic National Convention featured a lot of religious references. On the contrary, Democratic politicians praise religion all the time.
Why don’t liberals and Democrats go full-bore, “There is no God, it’s just us alone on a planet in a vast universe?”