My answer to your question would be a lengthy one as there many factors involved. But let me try to tell you some of that instead, just to give you a glimpse of it.
The warning was not meant for an specific individual but is for the church in general. In those days, and even today, not all who are called Christians or claim to be Christians and so are considered belonging to the church, are all necessarily legitimate children of God or are truly converted like Paul. If we know that, I see no reason why Paul would not know that. Besides, that fact is right up to his face. For he sees some, of not many, of them who are unstable with regards what they claim to have believed. Then we have the time context wherein, in those times, the gospel is yet to be better understood as preaching about it and knowledge about it is yet in its infancy so to speak.
Those, in my view, may be some of the reasons why not only that Paul gives warnings, but also reminders, and exhortations, but also still preach and continues to preach and explain the gospel more fully or deeply to the church, and not only preach to the world. I also see that as an effort of Paul to somehow make certain, that what those in the church have believed concerning Jesus Christ or the gospel, is the true, correct, spiritual knowledge and understanding that comes from the Father, Jesus Christ, and the Holy Spirit.
Tong
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