I was actually raised in a church that taught me that atheists were people who worshiped Satan

It's amazing that some self proclaimed religious experts can have a such poor understanding of theology.

From the standpoint of conservative Christians they believe that unless one undergoes the process of Justification and Sanctification one is at risk of their soul being condemned to hell. Other religions and atheism are a threat to them because in their view they have the capability of turning people away from God and straight into the arms of Satan. Atheism especially is a threat because even the most conservative and die-hard groups struggle with doubt. And while its a stretch to think that a die-hard Christian will become Buddhist or Hindu or Muslim, it's a very real threat to them that one could lose or renounce their faith altogether. Some view their doubts about their faith as the temptation of Satan, so external reinforcement of those doubts by atheists and atheist groups can also be interpreted as satanic in nature.

I don't know how you address this line of thinking. I don't know how you get through to someone who thinks their soul is under attack at all times by dark forces, and anyone who is not in strict alignment with their theology is likely an agent of the devil. It sounds a lot like a mental illness if you ask me, but then again I'm a part of the "Angry atheist" stereotype.

Edit: In the interest of full disclosure I wouldn't actually call myself an atheist. But I certainly don't believe in the God of Abraham. Even if I did his conduct as described in the Bible is deplorable; he would not come close to being worthy of my worship.