Actually, this didn't start with Christians, but with pagans, or
at least the Greek and Roman pagans, who had just as dim a
view of atheists as fundamentalist Christians. The considered
religion to be the foundation of society and morality (like
fundamentalist Christians), so they considered atheists to be, at
best, people without any moral compunctions. In fact, when
attacking early Christians, many pagans accused them of being
atheists because they denied the reality of the pagan gods.