Christianity has to do with faith in Jesus forgiveness. If you are an atheist, you certainly don't have this forgiveness. But that doesn't stop Christians from having the forgiveness. After all, that is what Christianity is all about... forgiveness. People who don't need forgiveness are not Christians. Christians need and get forgiveness because they do wrong things.
Christianity has to do with believing in Jesus salvation for the wrongdoings of the christian, not the wrongdoings of the atheist. Hell for Christians is impossible. Hell is for non-Christians, even though some of them say they are Christians.
You can't look at any heart. You can only guess from the actions. So, you can't determine for a fact that anybody is Christian-going-to-Heaven, or a faker going to Hell. But what you can determine from the fact of what Christianity is about, is that Christians are not going to Hell. But this doesn't help you determine who is a Christian in reality and who is a liar.
Regards,
-Joseph Van Name Ph.D.