Take a look at the first sentence of Chapter 6, something you might pass over as you get into the chapter without realizing its implications:
1 Working together with him, then, we entreat you not to accept the grace of God in vain.
Now that is a loaded sentence! The Corinthians are converts. They believe in Christ, they have already been converted by the Holy Spirit. But throughout this letter Paul is cajoling them to RETURN (or be reconciled) to unity and communion with him. Here he is stating his point directly. The Corinthians have accepted the grace of God but if they continue on this path they would have done so in vain.
Break down this simple sentence and look at all it shows us:
1. “Working together with him” – Paul says this elsewhere, we are God’s co-workers.
2. “we entreat you” – they are being made aware of their error. The Corinthians are being told that they are in error and being asked to change their ways.
3. The grace of God can be “accepted” – to accept is a choice. One can accept or not accept a gift. That is free will.
4. The grace of God can be accepted “in vain.” – They accepted, they were saved. Paul was there. He oversaw their conversion personally but they faltered and followed false preachers and started believing false doctrines and thus there acceptance will have been in vain, if they do not repent and be reconciled.
Thus this simple sentence establishes free will and excludes the idea of once saved always saved. It shows that even though one is saved, one can “loose” their salvation in that they can fall into error and sin AND once being made aware of their error and sin, they can choose to continue to follow error and sin and not be reconciled. THIS IS THE KEY POINT, mistakes or errors made in good conscience do not cause you to lose your salvation. HOWEVER, once you are made aware of you error you cannot persist in holding onto that error. This is the point Paul makes in his subsequent verses. He tells them that now is the time to be reconciled and he details all the hardships he has endured to be their example and spiritual father. Paul is cajoling them to be prepared to bare similar hardships to be reconciled.