How does Saint John distinguish the children of God from the children of the devil through righteousness and sacrificial love in 1 John 3?

Saint John begins by marveling at the boundless generosity of divine charity, inviting disciples to contemplate what extraordinary love the Father has lavished upon humanity by elevating believers not merely in name, but in authentic reality, to become the adopted sons of God. Because the secular world did not recognize Christ, it inevitably fails to comprehend the exalted dignity of His followers. The apostle reveals that while believers are already true children of God in this earthly life, the full splendor of their future state remains veiled. Yet Christians possess the certain hope that when the Lord is manifested in glory, they will be transformed into His likeness, because they will behold Him as He truly is in the beatific vision. Every soul that cherishes this transcendent hope is compelled to purify and sanctify himself in moral life, striving after the holiness of Christ Himself.

Contrasting this filial calling with moral rebellion, John defines sin as iniquity—what the annotation calls transgression of the law. The Son of God was manifested in human flesh for the precise purpose of taking away our sins, and within His divine person there is no sin whatsoever. Consequently, whoever abides in Christ sins not; the annotation specifies mortally; whoever surrenders to sin demonstrates that he has neither truly seen nor known the Savior. Warning disciples not to be deceived, the apostle insists that true righteousness is demonstrated by concrete righteous deeds, just as Christ is essentially righteous. Anyone who practices sin aligns himself with the devil, who has sinned from the beginning; indeed, the Son of God came into the world precisely to dismantle and annihilate the works of the devil. The soul that is spiritually regenerated by God possesses the abiding seed of divine grace, preventing him from living in sin so long as he guards that supernatural life, as the annotation explains. It explicitly adds that he may fall from this state by abuse of free will. Through this decisive moral contrast, the children of God and the children of the devil are laid bare: whoever fails to practice justice or refuses to love his brother does not belong to God.

The foundational message delivered from the inception of the gospel is the absolute command to love one another. Disciples must never resemble Cain, who belonged to the evil one and slaughtered his own brother simply because his brother's deeds were righteous while his own were wicked. Believers should not be astonished if a hostile world treats them with hatred. Rather, Christians know with joyful certainty that they have crossed over from spiritual death into supernatural life because they cherish sincere love for their brethren. Anyone who lacks love abides in spiritual death, and whoever harbors hatred toward his brother is morally a murderer, excluded from the inheritance of eternal life. The authentic nature of divine charity was definitively unveiled on the Cross, where Christ willingly laid down His life for our redemption; therefore, disciples are obligated to lay down their lives for their Christian brethren. On a practical level, if someone possesses earthly wealth and witnesses his brother in dire necessity, yet pitilessly shuts his heart against him, the love of God cannot possibly dwell within his soul. John commands his children not to confine their love to superficial words or eloquent speech, but to express it in tangible deeds and authentic truth.

Practicing active charity provides believers with interior peace and confidence in the presence of God. If the heart reprehends us, God is greater than our heart and knows all things; if it does not reprehend us, there is confidence towards God. When their hearts do not condemn them, disciples approach the Almighty with serene confidence, receiving whatever they ask in prayer because they keep His commandments and perform deeds pleasing in His sight. John summarizes the overarching divine commandment: to place lively faith in the name of His Son Jesus Christ and to love one another in accordance with His precept. Whoever faithfully observes God's commandments abides in intimate union with God, and God abides in him. The authentic proof of this mutual indwelling is confirmed by the Holy Spirit, whom God has generously bestowed upon His Church.