This is how God’s children and the devil’s children become obvious. Whoever does not do what is right is not of God, especially the one who does not love his brother or sister.
This introduces us to the concept that conduct exposes certain spiritual realities, and prepares us for John’s discourse on the supreme spiritual reality of love in those who have been born of God that will follow next.
“This is how God’s children and the devil’s children become obvious”: John has spoken with such clarity that he expects these truths to be obvious to his spiritual children. John expressly refutes the notion that you can live in Satan’s nature and belong to God. This is a spiritual impossibility because it denies the supernatural act of regeneration that all believers undergo when they are born again.
“Whoever does not do what is right is not of God”: Those who do not demonstrate the righteous life of God in their life are clearly understood as not being born of him. Although these early believers were facing an onslaught of theological deception that insisted Christians were not responsible for sin or righteousness, John reasserts that your lifestyle proves who you belong to.
This is the heartbeat of apostolic doctrine:
1 Peter 1:14–16 (CSB) — 14 As obedient children, do not be conformed to the desires of your former ignorance. 15 But as the one who called you is holy, you also are to be holy in all your conduct; 16 for it is written, Be holy, because I am holy.
The believer is exhorted by the unified witness of Scripture that they have been set apart to a holy God and must come to him in holiness as new creations by the grace of Christ, resisting the life that they have now put off, and being conformed to the glory of the one who has redeemed them from the powers of darkness.
“Especially the one who does not love his brother or sister”: This prepares us for the coming discourse on “love”, which John presents as the supreme experience of any authentic Christian faith.
And on this note I believe we hear something of supreme importance to our day: “if it is evident that sin reveals one’s Satanic origins, it is even more evident that those who do not love their brothers or sisters reveal their Satanic origins by their lovelessness.”
Many modern believers operate under the assumption that love is an optional attribute of the Christian experience. They assume that the total lack of Christ-like love expression in their lives is a trivial matter. But nothing could be farther from the truth! Those who live and spend their lives entirely on themselves demonstrate by their lovelessness that they do not belong to the one who lived and spent his life entirely on others!
The one who does not love their brother or sister in Christ demonstrates in the clearest of terms that they do not belong to God. In other words, “love” is not merely an optional attribute of Christian experience, but it is the essential attribute of Christian experience and one cannot claim to be born of Christ if one does not love their brothers or sisters in Christ!
One of the most common means of escaping this responsibility is to redefine who is a brother or sister in Christ. But John has already gone to such great lengths to identify who belongs to Christ that it is impossible to escape his conclusion; anyone who lives their life in the faith and practice of Christ belongs to God. This is the inescapable conclusion of John’s epistle. We cannot write anyone off who lives in the faith and practice of Christ because their life demonstrates that they belong to God.
This proposition will dominate the remaining two chapters of John’s first epistle and merits the attention he gives to it.
Biblical Reasons 
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