The Greek noun apostasia means apostasy, defection, or a departure from the faith. It comes from apo- (away from) and histemi (to stand), literally meaning 'to stand away from.' In the New Testament, it specifically refers to a rebellion against or abandonment of God's truth.
Apostasia is one of the most theologically sobering terms in the New Testament. Paul warns in 2 Thessalonians 2:3 that the 'day of the Lord' will not come until the apostasia — the great rebellion or falling away — occurs first. This points to a future large-scale defection from the faith. In Acts 21:21, it is used of the false accusation that Paul teaches Jews to forsake (apostasia) Moses. The word warns that genuine faith can be falsely professed, and that the end times will be marked by widespread departure from true Christianity. The antidote is the steadfast perseverance of genuine faith (Hebrews 10:26-39).