Atheists are usually secular humanists – focused on the immediate world of human experience. Pagans are more like Christians because they both believe that there is a spiritual realm outside of human experience.
I know, at first blush, it seems logical that atheists would be the ones who are more like pagans.
Instead of juggling definitions, it is easier to understand these relationships from an historical perspective. As I explain in my book, Perils of Empire, like most pagans, the Romans did not understand why many things happened – people fell ill and died suddenly, some soldiers survived many battles while others died in their first skirmish, winter came and went – so they felt that gods or spirits must control these events and did so without seeming to care much about the fate of mere individuals. Through ceremonies, the many different pagan cults acknowledged the power of these gods, requested their assistance, and gave people a way to express uncertainty about their fate. This went on for thousands of years and was deeply embedded in all human cultures.
As the Christian religion developed and grew, many Christian holidays and traditions were borrowed from the pagan ceremonies already prevalent in the ancient world. Appropriating pagan-like traditions (for example the Christmas holiday occurs very close to the pagan solstice holiday) made it easier for pagans to convert to Christianity.
In addition, one of the attractions of the new religion was the idea that unlike pagan gods, the Christian God cared about you as an individual. Note that, in The Iliad, the gods openly meddle in the war between the Greeks and the Trojans, playing out their own feuds with no thought about the consequences for humans. A God who was eager to provide human believers with life after death was a very positive difference.
In contrast to this ancient contest, modern secular atheists don’t have a lot of ceremonies or teachings or traditions. Most atheists are “humanists” in some way – simple truths about the dignity and value of human life form the basis of their ethical beliefs and behavior.
Now, some atheists are “spiritualists” who consciously take up pagan rituals to create ties with old cultures and with nature. However, modern spiritualists don’t literally believe that pagan gods or spirits exist outside of the human realm. Remember, ancient pagans felt that nature really was populated with gods who were independent of humans and did their own thing. Modern spiritualists are more descendents of Jung’s ideas about a spiritual connection between humans and other living things – some sort of collective unconscious – so in that sense they are not truly pagan.
As a result, I think Christians, who believe in a God who exists outside of the human realm, are more like pagans than either atheists or spiritualists, who have a more human-based ethical system. Keep in mind that both monotheism and polytheism are religious perspectives; their differences are over the number of Gods and their relationship with humans.