1 John 1:3

We proclaim to you what we have seen and heard, so that you also may have fellowship with us. And our fellowship is with the Father and with his Son, Jesus Christ.

(v3, NIV)

John has been talking about the word of life. He has been talking about Jesus Christ who lived in the flesh who has has seen and heard and touched. There are other witnesses and believers of Christ who have also seen, heard and touched him. These things these witnesses have seen, testify and proclaim. So why are they proclaiming it? John explains that witnesses are testifying to these things so that you can have fellowship with those who proclaim it. People proclaim the word of life so that those who hear can have fellowship with them. You can have fellowship with anyone you want to. However, in order to have genuine fellowship in the context of Jesus Christ, we have to be in one accord about Jesus Christ our Lord. Those who testify, proclaim and believe in the Lord Jesus Christ have fellowship with the father and with his son, Jesus Christ. So, if a believer has fellowship with God, how can he have true fellowship with you if you don't also have fellowship with God as he or she does? We have to be unified in one accord. The thing in which we have to be unified in is Jesus Christ. We have to have our own fellowship with God. If we have that, we can have true fellowship with one another.