According to Christian perspectives you mentioned, a person who claims that Jesus Christ was not in any way divine, and that his death was not spiritually meaningful in any way cannot enter into the Body of Christ, Community of Saints, or the Kingdom of God.
Consider the following soteriological perspectives,
Salvation by faith: regardless of other principles or teaching, most self proclaimed adherents to mainstream Christian religions require that a person acknowledges that they believe in the resurrection of Christ and some of its consequences.
Repentance from Sin: While most Christian groups acknowledge that salvation is by faith alone (Reformed, Evangelical, Catholic), some acknowledge that a person should have a life after conversion in which their behavior is altogether different than that of their pre-conversion life.
Decisionalism: Yet other Christian groups, and some of those also mentioned, teach that a person must make a conscious decision to follow Jesus, and that each should be able to point to a moment of realization of the truth and salvation.
Lordship: Others yet claim that Salvation comes when people turn their lives over to Christ, as their Lord and Master.
Sanctification: If a person obtains justification from sin by their sanctification, a person must become holy by living a biblically driven life, their sins being forgiven because of their dedication to godliness.
If an atheist does not fall into any of these categories, most of the Christian groups that operate according to any one of these doctrines, including the Reformed, Evangelical, Catholic, or Orthodox churches, as well as others like Mormonism, would not include such a person among those who, according to many of these groups, will continue to live eternally in communion with God.