I would suggest that there are some who affirm the Doctrines of Grace who were brought into the visible church through a false gospel. And that such may not yet have been granted faith in the imputed righteousness of Christ alone for salvation. These do not yet see that they are truly deserving of hell. And having come to a Jesus who "died for all", longs to save "all" and yet is powerless to save "any", have put their faith in a false Jesus.
These false converts are intellectually convinced of the Doctrines of Grace but have never come to genuine repentance or faith in the imputed righteousness of Christ alone for salvation.
I would suggest that this is why some who affirm Calvinism, view Calvinism, as merely a "system" and do not see a rejection of the imputed righteousness of Christ alone as a rejection of the gospel itself.
I would call these "intellectual Calvinists". In my experience they reason that they were truly born-again under a false gospel and that they simply came to the truth in a normal gradation that all Christians must go through. This would help explain why they view belief in a false gospel as merely, "immature Christianity" and not counterfeit Christianity.
I believe that this was who I was. I was brought into the visible church through a false gospel. Though I became an "Intellectual Calvinist" shortly there after, I was not born-again until much later.
As a Calvinist I could mimic the words of other Calvinists but I never saw myself as truly deserving hell. I never surrender the throne to God and was never truly subject to the word of God, I would suggest that this was one reason I could "Popishly" declare those believing in a false gospel to be saved.
(Funny thing - even though I proclaimed them to be saved, I always knew they we trusting in works to save themselves, but did not really think it was a big deal because I could list off dozens of famous Calvinists who declared them to be saved as well and honestly that was more important to me than the truth).
I believe that it is possible that a great many who affirm the Doctrines of Grace are merely intellectual converts to a system of theology and are not truly born-again.
I hope that God might see fit to use my experience as an "Intellectual Calvinist" to help others to consider the possibility that just because they affirm the Doctrines of Grace does not necessarily mean that they are born-again.