Reformed types (aka Calvinists, Presbyterians, etc) rightly believe in the doctrine of Perseverance of the Saints, aka Once Saved Always Saved. They rightly believe it because the Bible teaches it.
Jesus saves people and he doesn't fail to finally save those whom he calls and justifies.
Yet there is the reality of apostasy. Apostasy is the phenomenon of people in the church who profess faith, exhibit what appears to be evidence of salvation, receive the sacraments, and yet go on at some point to
deny the faith,
leave the church, and
fall away from Christ.
The Bible recognizes this phenomenon and is replete with warnings against falling away. Sadly we also experience this phenomenon in our lives with those who belong to our churches and those whom we love.
Many think that the reality of apostasy and the doctrine of OSAS are incompatible. But this is not what the Reformers taught. We must reckon with two profound, Biblical truths simultaneously:
- Those who are truly saved cannot ultimately fall away from salvation.
- It is possible for any Christian at any time to apostatize and we must diligently strive to keep the faith until the end.
There are many wrong ways to try to reckon with these truths. The most obvious error is to deny one or the other. Many Arminians end up denying (1) while many Calvinists end up denying (2). But the Bible teaches both. The Reformers understood this very well. How do you hold these two truths together?