SCOTUS ruled that Secular Humanism is a religion over twenty years ago. Since atheism is a primary tenet of Secular Humanism, I wonder why it took so long for this decision?
One takes a definitive stand on a theological issue. The other trusts science in lieu of theology.
A secularist might suspect that there is a God, but just denounce human efforts to learn about God as misguided. An atheist might find profound importance in the great religions (I exclude Islam here), while remaining certain that God does not exist.
In population terms, there are far more secularists than atheists. Most Democrats instinctively trust science, the federal government, main stream media and the faculty at the great Universities above all else. Few Democrats are certain that there is no God. Most Democrats are secularists, instinctively, even if they don't know what the word means. Few Democrats are atheists -- not that their belief in God guides their life, but that they don't actively disbelieve.