We shall be like him

Early Christians were lucky because they were able to live and mix with witnesses of the risen Jesus. The impact must have been so tremendous that these witnesses were willing to give up their life for what they had seen, touched and heard. The author of the first epistle of John (for convenience's sake, I will simply call him John because even if he was not John, he was writing in the authority of John) must be one of these witnesses, though we do not know for sure whether he suffered martyrdom.
That which was from the beginning, which we have heard, which we have seen with our eyes, which we have looked upon and touched with our hands, concerning the word of life (1 John 1:1).
Nowadays, we are far away in time from these witnesses and apostles. Are we less lucky than those early Christians? Not necessarily, because we believe in Jesus and he sends his Holy Spirit to teach us everything we need to know for our salvation.
the anointing which you received from him abides in you, and you have no need that any one should teach you; as his anointing teaches you about everything, and is true, and is no lie, just as it has taught you, abide in him (1 John 2:27).

John had been writing to the community to fight against a heresy which denied that Jesus is Christ. Now, he moved on to warn the community members from committing sins. If they did, they forfeited God's love, forfeited their identity as children of God (1 John 3:1). In this epistle, John constantly called them "children". There are two possibilities. John was aged and treated the members like his children. The other possibility was that he affirmed their identity as God's children.
See what love the Father has given us, that we should be called children of God; and so we are. (1 John 3:1a)
Being God's children, they had to resist the temptation to follow the world and its lust. If they did, they lost the love of God (1 John 2:15-17).
The reason why the world does not know us is that it did not know him (1 John 3:1b).
This reminds us of what Jesus said in the Last Supper and the close affinity between the gospel of John and this epistle.
O righteous Father, the world has not known thee, but I have known thee; and these know that thou hast sent me (John 17:25).
Beloved, we are God's children now; it does not yet appear what we shall be, but we know that when he appears we shall be like him, for we shall see him as he is (1 John 3:2).
John must have seen the glory of Jesus. He wanted his children to be like this glorified Jesus. To encourage them to abide in Jesus, to remain pure, to stay away from sins, John shared his vision of this glorified Jesus.
You know that he appeared to take away sins, and in him there is no sin.
No one who abides in him sins; no one who sins has either seen him or known him (1 John 3:5-6).
John's constant advice: to abide in Jesus and do not sin. We shall be like him when he appears.
Thank you John. Thank you for your loving advice.

Dear Jesus, keep us always in you from the evil one. Guard us and none of us is lost (John 17:12). Amen.