“In this the children of God and the children of the devil are manifest: Whoever does not practice righteousness is not of God, nor is he who does not love his brother.”
The words “in this” refer not only to the previous discussion about the sin nature and the divine nature but also bridges to the next section (3:11-24).
The child of God has the divine nature at the core of his being. This nature directs his orientation to life. The non-Christian displays his true nature through sin and man-made righteousness but the Christian cannot. The Christian cannot sin from the source of his divine nature because sin and God are mutually exclusive.
A tree is known by its fruit.
Only by fellowship with the Lord can we manifest the reality of Christianity to a lost and dying world. If we have hatred toward others or if we resent them, we cannot manifest the filling of the Spirit. The world cannot see the power of God in us because our sin blinds their minds to the reality of God’s work in our lives. There is no difference between our behavior pattern and that of a non-Christian in this case. Religious non-Christians behave morally.
The Christian is the letter that God writes to the world. Fellowship with God cannot but manifest itself to the world. He writes this letter not in the mere dead words of the law but in the power of the Spirit.
2 Co 3: 2 “You are our epistle written in our hearts, known and read by all men; 3 clearly you are an epistle of Christ, ministered by us, written not with ink but by the Spirit of the living God, not on tablets of stone but on tablets of flesh, that is, of the heart.”