There is a big difference between being ignorant, and being foolish, or stupid. To be ignorant simply means that one has not learned something—it is foreign to him. To be foolish, or stupid, is to be shown the truth about something, but reject it. Ignorance is not having learned something, but foolishness and stupidity are a choice to reject truth once it is learned.
Many christians are ignorant of the truths of the Holy Spirit, because they have never been taught what they are, and have been taught, rather, counterfeits of His truths by christianity. Now, when someone is shown a spiritual truth, but rejects it outright, just because it disagrees with the false teachings he has previously believed, that person is foolish, or stupid.
Being ignorant comes from simply not ever having learned something. Being stupid, or foolish, comes from hearing the truth, but rejecting it.
So, when the truths of the Spirit regarding the three spiritual harvests are shown to someone, but they reject those truths because they never learned them before, that’s a choice to be foolish. Merely not knowing those truths is just ignorance—but that can be fixed by learning.
Foolishness, or stupidity, is a choice.
Hearing the truths of the Spirit regarding salvation not being something man can declare, and not being a decision of man’s, and then rejecting those truths because they don’t line up with the teachings of christianity, is foolishness, because it is a choice not to believe the Holy Spirit’s truths.
Matthew 7:21-23 shows us many people who are shocked that what they believed was not the same as being born again.
For more information: Why I Am No Longer a Christian