The whole of John’s first epistle exhorts those who call themselves Christians to inspect themselves to check they are indeed what they say they are. It can be very easy to think we are something which we are not.
In this short paragraph, John says Christians are those in whom the Spirit of God lives. If the Spirit lives in us, we must know what God has done for us in his great love. For if we do not know the forgiveness of sins through his Son, how can we profess to be Christians? To know the forgiveness of sins, we must know the truth of the gospel and we must believe in the truth.
If we know the love of God through his Son, then we also must love God and in doing so do what he says, obeying his commands, including loving others. In this way, we know we are born of God, and what we claim to be.
Come to me, all you who are weary and burdened, and I will give you rest. Mt 11:28
After Joshua had conquered Canaan, his people could rest. Similarly, if Jesus conquers sin in our lives, we have rest. This is not to say we do nothing, but that we are no longer fighting ourselves.
For we are justified through faith in Christ, giving us rest from trying to make ourselves righteous and a rest from fear. Once we are justified, we undergo a process of sanctification, changing us from people who chase after the things of this world and never being satisfied, to those who seek the kingdom of God and finding our rest in God. Ultimately, we will go to eternal rest, not to be idle, but to rest from pain and suffering and to eternally praise and glorify God. For eternal life is to know God and to be with him.