Ques. Does it mean in 1 John 3:10 that every unconverted person is a child of the devil?
Ans. This Epistle contrasts the old nature of the flesh, which we received from our first parents, with the new eternal life communicated to us, and which is seen in all its perfection in our Lord Jesus Christ. Some of the contrasts are,
God contrasted with the devil.
Light contrasted with darkness.
Love contrasted with hatred.
Obedience contrasted with sin, lawlessness or disobedience.
Righteousness contrasted with unrighteousness or wickedness.
Life (eternal) contrasted with death.
The one belongs to the divine nature. Without it, all mankind are alike. Sin is the only fruit the flesh can bear. We were morally like the devil who sinned from the beginning. Such truth is disagreeable to our pride.
The blessed Lord is the only exception. "In Him is no sin," and all He did was the Father's will. His one obedience has brought all believers into blessing. To Him is all the glory. Wonderful grace!
Now this "old commandment" (Christ the eternal life) is a "new commandment," because it "is true in Him and in you" (See chapt. 2:7, 8). The believer has now a life whose character is morally divine, but the flesh in us now hinders its full development as it was manifested in Christ. What bliss it will be when rid of this hindrance, and we will be with Him and like Him forever!