Net Chaplain, the warnings are given to believers. An unbeliever cannot fall into unbelief. They are already there.I do not interpret (nor do some Bible commentators) verse 20 to be descriptive of believers but of those who were attempting to be accepted by believers. The confirmation of this is the fact of their finale lifestyle manifesting no change, like the dog and the pig in verse 22, always returning outwardly to who and what they still are. The "escaping" speaks only to a false sense of believing the Lord Jesus, for acceptance and conscience sake (Mat 23:28).
Considering the preceding verses describe these in verses 20-22, I wouldn't think it sensible to consider them accepting anything beyond only knowledge of Christ, esp. not receiving faith in Him, or He would have changed them like all the others who believe.
A Barnes - "For if after they have escaped the pollutions of the world" - This does not necessarily mean that they had been true Christians, and had fallen from grace. People may outwardly reform, and escape from the open corruptions which prevail around them, or which they had themselves practiced, and still have no true grace at heart.
John Gill (1697–1771) "Through the knowledge of the Lord and Savior Jesus Christ" - Neither does this imply that they were true Christians, or that they had ever had any saving knowledge of the Redeemer. There is a knowledge of the doctrines and duties of religion which may lead sinners to abandon their outward vices, which has no connection with saving grace. They may profess religion, and may know enough of religion to understand that it requires them to abandon their vicious habits, and still never be true Christians.
"Through the knowledge of the Lord and Saviour Jesus Christ." by which "knowledge" is meant, not a spiritual experimental knowledge of Christ, for that is eternal life, the beginning, pledge, and earnest of it; but a notional knowledge of Christ, or a profession of knowledge of him, for it may be rendered "acknowledgment"; or rather the Gospel of Christ, which, being only notionally received, may have such an effect on men, as outwardly to reform their lives, at least in some instances, and for a while, in whose hearts it has no place.
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