1 John 3:23-24 And this is his command: to believe in the name of his Son, Jesus Christ, and to love one another as he commanded us. Those who obey his commands live in him, and he in them. And this is how we know that he lives in us: We know it by the Spirit he gave us.
The Apostle John was writing these words to people who were identifying themselves with the followers of the Lord Jesus Christ, but whose lives and lifestyle were inconsistent with their profession of faith. His letter is included in the Canon of Scripture as a challenge to all of us to take careful stock of our own profession of faith. As Paul wrote to Timothy, “All Scripture is God-breathed and is useful for teaching, rebuking, correcting and training in righteousness, so that the man of God may be thoroughly equipped for every good work.” (2 Timothy 3:16-17).
John is well aware of the fact that we ARE ALL sinners: “If we claim to be without sin, we deceive ourselves and the truth is not in us. If we confess our sins, he is faithful and just and will forgive us our sins and purify us from all unrighteousness. If we claim we have not sinned, we make him out to be a liar and his word has no place in our lives.” (1:8-10). But the point he is making in his letter is that believing IN Jesus does something to CHANGE us and our attitude to sin in our lives. The only way we can really come to KNOW Jesus as our personal Saviour is by first realising that we are sinners who NEED a Saviour. (This is why the Pharisees rejected him as the Saviour!) When we DO come to ‘believe in the name of [God’s] Son, Jesus Christ’, and the struggle against the sin in in our lives becomes a reality, we realise more and more what it means to be a sinner-saved-by-GRACE-alone.
Before we take this life-changing step our default disposition is to just shrug our shoulders and to identify with the maxim that ‘to err is human’ and is therefore of little consequence. But when the Holy Spirit begins his gracious life-long work of sanctification in us, our whole attitude to sin must change. With Christ now living IN us, and we IN HIM, we realise that there is no place for our sin in this relationship, and so we are no longer willing to just let it take its natural course in us – we don’t ‘keep on sinning’ as if it doesn’t really matter (see verses 5-6: “But you know that he appeared so that he might take away our sins. And in him is no sin. No-one who lives in him keeps on sinning. No-one who continues to sin has either seen him or known him”).
Our state of being SAVED, of being secure in a right relationship with God for all eternity, is not based on how well we perform in this battle against sin, it is based only on our BELIEVING IN his NAME (cf 1:9-10, see above). BUT, notice what John says: “This is HOW we KNOW that he lives in us: We know it by the Spirit he gave us.”, and the Spirit’s work is to “convict the world of guilt in regard to sin and righteousness and judgement: …” (John 16:8)! Do you KNOW him in your life?