The expression "I know them" is very important and is already found in the Old Testament, as in Ex 33:12, 17 and Is 43. The expression "the Lord knows by name" is equivalent to this: I have called you because your soul is worthy to draw near to Me.
This was said of chosen instruments of God in the Old Testament. But in the New Testament the Lord calls every human soul to draw near to Him. Every human soul became worthy to draw near to the Lord only after it had been redeemed and cleansed by the mercy of God.
Therefore we must compare the Lord's words, "I know them," with the prophecy of Isaiah. "Now thus says the Lord to saved Israel: Fear not, for I have redeemed you. I have called you by your name; you are Mine."
Naturally, as we have often said above, only those souls follow the Lord's call which do not resist the voice calling them by an evil will. These alone are the Lord's sheep, not only by calling, but also by their own human free choice. The precious gift of God, human will, is never taken away from man.
We must note that in some Western teachings this passage of John the Theologian is interpreted as indicating "predestined salvation." This theory leads to quietism, reaching negligence and passing into the Muslim belief about predestination.
But the Lord promises salvation to the sheep who remain in His flock, not to those who fall away from the flock. The grace given by the Lord to the Church as the assembly of believers acts eternally. Yet one must desire it and thirst to use it, in order to receive it.
Those who neglect this gift are not the Lord's sheep. "The opponents of the Lord Jesus Christ," says St John the Theologian, "went out from us, but they were not of us. For if they had been of us, they would have remained with us. But they went out, that it might be revealed that not all are of us" (1 Jn 2:19).
It is a great gift to have the assurance that, remaining faithful to Christ and abiding in His Church, we shall receive salvation. But we must "work out" this salvation "with fear and trembling," as the Apostle teaches (Phil 2:12), and serve God with reverence (Heb 12:28).