Those saved always have knowledge, but not all those with knowledge are of the saved.
Those saved always have true faith, but not all those with knowledge have true faith.
Many may have superficial knowledge of Christ through their own human intellect, and some may even believe for a short while, but should they fall from it (and they will), their faith was not a true faith, and they, therefore, could never have been of the saved.
Peter said as much in the salutation of 2 Peter 2:1 (same book). He tells us in it that he is addressing those who have true faith.
How did they get it? By their knowledge? Nope! But by the righteousness of Christ and not of themselves. And if by Christ's righteousness, then it cannot be lost by man's unrighteousness.
[2Pe 1:1 KJV]
1 Simon Peter, a servant and an apostle of Jesus Christ, to them that have obtained like precious faith with us through the righteousness of God and our Saviour Jesus Christ: