Apostle John wrote in 1 John 1:3 "Our fellowship is with the Father". True fellowship is in two directions, like the two arms of the cross. It is through the cross that we come into fellowship with God and also with one another. Between Christ and us there is a cross on which He died. Because of that we can have fellowship with our Lord. We can never have fellowship with God apart from that, because in ourselves, we will never be good enough. And between us as believers also, there must be a cross on which each of us die to ourselves, if we want to have fellowship with each other. Fellowship is impossible without this death on the cross - both in the vertical and in the horizontal directions. The cross is the secret of life and fellowship. There is no life without the cross and there is no fellowship possible without the cross. The cross was in God's mind from way back in eternity past. The Lamb was slain "from before the foundation of the world" (Revelation 13:8 - KJV). God knows the end from the beginning and so the Trinity knew from all eternity past, that the Second Person would have to come to the earth as a Man and be crucified for man's sins. That was not something God decided on, after Adam sinned. It was eternally foreknown. God placed a sword in front of the tree of life when Adam sinned. That sword fell on Jesus and slew Him. And that sword must fall on our Adamic life too - we must accept our place as "crucified with Christ" (Galatians 2:20) - if we are to come to the tree of life and have fellowship with God and also with each other.
John says that fellowship will result in fullness of joy (1 John 1:4). Joy is a major part of the Christian life, because joy is the atmosphere of heaven. There is no gloom in heaven. The angels are never discouraged. They are always full of life and full of joy. And we can have that joy too, if we have fellowship with God. The Holy Spirit has come to bring the atmosphere of heaven into our hearts - and part of that is a fullness of joy. Satan will tell you that if you give your life totally to God, you will be miserable, gloomy, depressed, and long-faced. It is unfortunately true that some Christians do give that impression by their appearance. I heard a story of a long-faced Christian witnessing to someone and asking him, "Would you like to receive Christ into your heart?" That person looked at the Christian's face and replied, "No, thank you. I have enough problems already!" That is a poor witness for our wonderful Lord. If your life and your home are not radiating the joy of the Lord, then something is wrong in your life. You have missed the will of God somewhere.
John goes on to say that if you want this life, fellowship and joy, then the very first thing you need to understand is that God is light and that there is no darkness in Him at all (1 John 1:5) - zero lying, zero impurity, zero hatred, zero pride, etc. Do you want such a life where you never tell a lie, never hate anyone, where you are never jealous of anyone and never proud? If you choose that life, you will never be gloomy or discouraged. You will live a life of perpetual joy in the Lord. Is it possible to live such a life on this sin-cursed earth? Yes it is. Philippians 4:4 commands us to rejoice in the Lord always. That was written for people here on earth and not for those in heaven! Your joy can be full here on earth, whether you are being persecuted like John on Patmos, or whether you are sitting comfortably at home. Your joy will never be dependent on your circumstances, if you choose to walk in the light of God at all times.
But if we say we have fellowship with Him and we walk in darkness, then we are not practising the truth. There are many Christians who say that they have fellowship with God but walk in sin. You can see in their faces that the joy of the Lord is not there. There is no spring in their step, no song on their lips, and no sparkle in their eyes. They have missed the joy of fellowship with God. The older we become as believers, the more joy we will have, if we are walking with God.