I am a graduate of Calvin and have a child there currently who is a CS major.
Calvin is a very Christian college. All of the professors and many of the students take that very seriously. However it is also very committed to academic rigor and intellectual questioning of everything.
As an atheist you would be welcomed with open arms and I don’t think that would hinder your social life. But, and this is a big but, you would probably find yourself constantly challenged to defend your beliefs (or non beliefs). The assumption at all times will be that you are a Christian. Dating could be an issue as many if not most students will be uninterested in dating someone who isn’t a Christian.
As you noted, Calvin has a lot of awesome things going for it and I’m really pleased with the CS department but you need to think hard about how you feel about being in a pervasively Christian environment.