Christians you believe that the evil one could make an abode in the atheist, but what about in you?

I'm sorry, Shinigami, but the bible doesn't teach this. You're moving into a highly speculative area here and the only reasonable navigation is on a case by case basis, each step guided by the Holy Spirit. In my experience, most western Christians don't go around looking for demons behind every rock (some do), or inside every atheist. Demonization seems to be more prevalent in areas of overt idolatry, if one can believe the stories one hears (always a dubious proposition, granted), but even there, demonization is most likely the extreme exception rather than the rule.