The once saved always saved doctrine (e.g. OSAS) teaches that once someone has been saved they can never fall away or “fall from grace.” This argument usually stems from an absolute position regarding God’s sovereignty. The reasoning goes something like:
If God is sovereign (which He is), and God wants all to be saved (which He does), then there is literally nothing that can thwart His will, including man’s own will or desire.
The once saved always saved doctrine permeates throughout modern Christian thought and practice. It is a tenant of Calvinism (Perseverance of the Saints) and Reformed theology.