As they started using Christian as synonym for good, those same people were so certain that worshipping God was necessary to be a good person, that atheist took on a meaning as being a bad person. That it is impossible for an atheist to be good.

A Catholic official actually stated that a person without religion cannot be moral and thus cannot be in a moral society. This was in 1995.

It was a big part of the demonization of "godless Communism", despite the fact that there was no such thing as a communist country, only communist parties.

Again, as stated in other writeups, "good" is defined as emanating from God; "evil" is that which is Not Good. If someone is equating "atheist" with "evil", that's their rationale. See morals and atheism for the morality thing; the catch-as-catch-can morality of secular religions doesn't cut it - look outside your window.

A prime example of "If you are not with us, you're against us" mentality. This is generally the reason why non-Christians feel alienated by "the church" and thus display animosity toward it. Note that atheists and agnostics rarely attack Jews or Buddists; thus the circle of negative feelings continues due to an unrelenting and immature view of non-Christians by many Christian groups.

Pingouin also brings up an interesting point. The U.S. is rooted in Chistianity and thus geared unfairly against people who do not feel the same way that Christians do. Obviously, true justice for all is merely an ideal and not a reality or even a possibility, yet the scales are tilted almost comically towards Christian Ideals. Hopefully some day we will have a true division of church and State.

One of the problems with this 'anything christian is good and anything athiest is evil' idea is not so much in how people expect their peers to behave, but how they expect people of authority to behave. People expect politicians, teachers and popular leaders to be 'good people', and thus they are expected to hold strict Christian beliefs.

Thus a politician who declares themself to be an athiest is unlikely to become popular, because they will be seen as being slightly evil, or at least amoral. Even a politician who declares themselves Christian can be suspected of being slightly 'unchristian'.

This in turn hurts liberal and other non-christian beliefs such as the pro-choice stance and homosexual rights. This also suggests why creationism is more popular than one would expect; anyone speaking out against it is declaring their own beliefs in the falisity of the bible.

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.