‘By this we will know that we are of the truth, and will convince (persuade) our heart before him, because if our heart condemn us, God is greater than our heart, and knows all things.'
To the modern commentator these and the following verses are seen as somewhat linguistically complicated. This may possibly arise from the Greek spoken in John's environment. What may seem to us complicated Greek may merely be colloquial. But whatever the case we must do what we can with it.
‘By this' must surely refer back to the previous verse, for it makes little sense to look for an application for it in what follows. It is by being genuine in the outworking of love for our fellow believers in the truth that we can know that we are of the truth. It demonstrates our love for the truth.
‘And will convince (persuade) our heart before him.' John expects that his letter will have made his readers and hearers ask themselves personal questions about their own state. He knows that their consciences will be at work. That is part of the purpose of his letter. So he wants to give them some assurance. Having examined themselves he wants them, if they are genuine Christians, to be convinced in their own hearts that all is well with them.
So he points out that once they can be satisfied that their love for their brethren who hold to the truth is genuine, and being genuinely lived out, they can know that they are of the truth, and can therefore convince themselves that all is right between them and God (‘right before Him, that is, in His presence') in their hearts and consciences in His presence (‘Heart' indicates the whole inner man, including reason, will, conscience and emotions). The point here is not that love for the brethren saves, but that it reveals that they are within the flow of truth, that they love the truth. It is among the true brethren that the truth is held and preached, and to love them and not the false prophets is to demonstrate a willing acceptance of the truth they teach.
‘Because if our heart condemn us, God is greater than our heart, and knows all things.' The question here is as to whether this is intended to give further assurance, that even if we still doubt (a sign in this case of a tender conscience rather than a lack of love), our genuine self-examination is evidence that we are genuine, and we can therefore remember that God knows all things and will therefore still accept us, so that our hearts can finally be convinced even in the midst of doubt. We can take comfort from God's all-knowing and know that He knows the genuineness of our faith. Or whether it is a caveat entered on the basis that God knows all things and knows how we really think and are, and suggests therefore that we need to look to ourselves. The context suggests that the first is in mind. John wants God's people to have assurance. But it may be that he wanted it to be ambiguous in order to meet different cases. The one of tender conscience to take comfort from it. The more hardhearted to be made to think.
Christian confidence is a theme of John. He has previously spoken of coming to God in confident confession (1 John 2:1), of having confidence before God at the coming of Christ (1 John 2:28), and he later speaks of having confidence at the judgment (1 John 4:17), and of the confidence believers have with God when they pray (1 John 5:14-15). This would support the idea that his aim is to plant confidence here. He seeks for Christians to be confident if they have true grounds for confidence.