VERSE 4: A Liar
Whoever says, “I know him,” but does not do what he commands is a liar, and the truth is not in that person.
This verse begins unfolding the second proposition of John’s first epistle in verse three.
John assumes the most fundamental realities of the nature of truth:
Truth liberates the believer:
John 8:31-32 (CSB) Then Jesus said to the Jews who had believed him, “If you continue in my word, you really are my disciples. 32 You will know the truth, and the truth will set you free.”
Truth sanctifies the believer:
John 15:3-6 (CSB) You are already clean because of the word I have spoken to you. 4 Remain in me, and I in you. Just as a branch is unable to produce fruit by itself unless it remains on the vine, neither can you unless you remain in me. 5 I am the vine; you are the branches. The one who remains in me and I in him produces much fruit, because you can do nothing without me. 6 If anyone does not remain in me, he is thrown aside like a branch and he withers. They gather them, throw them into the fire, and they are burned.
John’s language parallels 1st John 1:6-7 and continues explaining the contrast between walking in the light versus the darkness by showing us that “not keeping His commands” is what it means to “walk in the darkness”.
“I know Him”: this explains what was meant by “fellowship” in 1:6 and continues to explain the eternal life (1:2) that John’s first epistle is declaring:
John 17:3 (CSB) This is eternal life: that they may know you, the only true God, and the one you have sent—Jesus Christ.
Whoever knows Christ in the fellowship of his light will not walk in the darkness of death because the atoning sacrifice that John explained in 2:1-2 has restored their broken relationship with God through the advocacy of Christ.
Here it is important to note that our fellowship with Christ is the means by which we walk in the light and experience eternal life. Our restored relationship with Christ is the virtue by which we keep his commands; believers cannot keep his commands unless their personal relationship with God has been restored through the atoning sacrifice of Christ and his ongoing ministry of advocacy on our behalf in Heaven.
“A Liar”: whoever claims to know Christ but does not walk in his way has denied the fundamental reality of fellowship with Christ. It’s not possible to experience fellowship with Christ in the darkness because there is no darkness in God (1:5). When Jesus restores the believer’s relationship with God through his atoning sacrifice, he transfers that person out of the darkness and accompanies them into the light:
Colossians 1:13 (CSB) He has rescued us from the domain of darkness and transferred us into the kingdom of the Son he loves.
Jesus wants us to understand these realities because it is through the power of this gospel that sinners are saved, but many walk as enemies of the cross being deceived by false assurances that they are saved:
Matthew 7:21-23 (CSB) Not everyone who says to me, ‘Lord, Lord,’ will enter the kingdom of heaven, but only the one who does the will of my Father in heaven. 22 On that day many will say to me, ‘Lord, Lord, didn’t we prophesy in your name, drive out demons in your name, and do many miracles in your name?’ 23 Then I will announce to them, ‘I never knew you. Depart from me, you lawbreakers!’
Philippians 3:18-19 (CSB) For I have often told you, and now say again with tears, that many live as enemies of the cross of Christ. 19 Their end is destruction; their god is their stomach; their glory is in their shame. They are focused on earthly things
Many are placing their confidence in ecstatic signs or counterfeit grace but are living completely separated from a restored relationship with Christ. Their lives are filled with malice, envy and greed, hate, bitterness, and all kinds of sexual immorality, by which they show that they do not know Christ.
Biblical Reasons 