Because the Father showed His love by sending His unique Son so that we might live through Him, and by making Him the atoning sacrifice for our sins, those who are born of God must love one another. Along with the Spirit’s gift and true confession that Jesus is the Son of God, this love shows that a person is abiding in God. Where God’s love reaches its intended goal in believers, it brings confidence for the day of judgment, but anyone who claims to love God while hating a fellow believer is shown to be false.

John commands believers to love one another, and he immediately gives the reason: love is from God. Everyone who truly loves gives evidence of having been born of God and of knowing God. On the other hand, the one who does not love does not know God, because God is love.

That statement, "God is love," is defined by what God has done. John does not allow us to fill the word with human sentiment or vague kindness. God showed His love by sending His one and only Son into the world so that we might live through Him. Then John states it even more sharply: love is not that we first loved God, but that He loved us and sent His Son as the atoning sacrifice for our sins. God’s love, then, deals with sin, guilt, and the judgment to come.

From this John draws the clear moral conclusion: if God so loved us, we also ought to love one another. This is not optional for some higher class of Christians. It is the necessary expression of those who have been born of God. No one has seen God, but if we love one another, God abides in us and His love is perfected in us. In this way, the unseen God makes His indwelling presence visible through the love of His people. Here, “perfected” does not mean sinless perfection. It means that God’s love is reaching its intended goal in believers.

John then gives the grounds of assurance in this section. God has given us of His Spirit. The apostles have seen and testify that the Father has sent the Son to be the Savior of the world. This does not mean that every person is automatically saved. Rather, it points to the broad scope and full sufficiency of the Son’s saving mission, while the passage still insists on confessing the Son. So John adds that whoever confesses that Jesus is the Son of God abides in God, and God in him. The Spirit’s gift, apostolic testimony, confession of the Son, and mutual love therefore belong together.

John says that believers have come to know and believe the love God has for them. Again he says, “God is love,” and the one who abides in love abides in God, and God in him. In this context, that means abiding in the God-defined love revealed in the sending of the Son and expressed in obedient love for fellow believers.

When this love is perfected with us, it produces confidence for the day of judgment. John says, “because as He is, so also are we in this world.” In context, the point is that believers share Christ’s accepted standing before the Father, and therefore may face judgment with confidence in view. John is not saying that believers are already morally identical to Christ in every respect.

So there is no fear in love, but perfect love drives out fear, because fear has to do with punishment. John is speaking specifically about dread of punitive judgment, not about every kind of emotional fear or anxiety. The one who is still ruled by that dread has not yet been perfected in love.

We love because He loved us first. God’s prior love is the source of all true Christian love.

Finally, John applies the test plainly: if anyone says, “I love God,” and hates his fellow believer, he is a liar. If a person does not love the brother he has seen, he cannot love God whom he has not seen. Love for the unseen God is tested by love for visible brothers and sisters. Therefore the command still stands: the one who loves God must also love his fellow believer.

This passage offers both assurance and warning. It assures those who rest in God’s prior love shown in the sent Son, confess Jesus truly, receive the Spirit, and love fellow believers. It warns that loveless claims to know and love God are false. John will not allow profession without truth and visible love.

Key Truths: - Love for fellow believers is a necessary mark of being born of God. - “God is love” must be understood through the Father’s sending of the Son for life and as the atoning sacrifice for sins. - God’s love is prior and initiating; our love is a response to His love. - The Spirit’s gift, apostolic testimony, confession of Jesus as the Son of God, and mutual love all function together as marks of abiding in God. - Perfected love means God’s love reaching its intended goal in believers, producing loving obedience and confidence for the day of judgment. - The fear in verses 17–18 is fear of punishment, not every form of fear or anxiety. - Whoever claims to love God while hating a fellow believer is exposed as false.