Eternal security, also known as "once saved, always saved," is the belief that from the moment anyone becomes a Christian, they will be saved and will not lose salvation. Once a person is truly "born of God" or "regenerated" by the indwelling of the Holy Spirit, nothing in heaven or earth "shall be able to separate (them) from the love of God" (Romans 8:39), and thus nothing can reverse the condition of having become a Christian. Is this true? Hebrews 6:4-6 is a major proof text either way.
APOSTASY: Abandoning, Deserting, Working Against and Rebelling Against God
An apostate's defection from the faith may be intellectual, as in the case of Ernst Haeckel, who, because of his materialistic philosophy, publicly and formally renounced Christianity and the church. Then, there is Dr. Bart D. Ehrman who has rejected Christ because of his doubt in the trustworthiness of the Bible and the problem of evil, and so he has actively worked in opposition to Christ for 20+ years. It may also be moral and spiritual, as with Judas, who for filthy silver pieces basely betrayed his Lord.