John wrote from his personal, firsthand, experience with the Lord Jesus Christ. His desire was that we would have fellowship with God the Father, with his Son Jesus Christ, and with believers in the Lord Jesus Christ.
It’s possible to have fellowship with people who profess Christ and not have fellowship with God. This is the case in most churches around the world. God’s not present in their fellowship.
However, you can’t have fellowship with God without having fellowship with “us”, 1 Jn 1:3, if we are believers walking in the light. At some point, the Lord is going to draw you into fellowship with other believers in a solid local church. If you think God wants you to walk in the light with him and not walk with his children who are walking in the light, you are mistaken.
John is urging us to have fellowship with both the Lord and “us” (believers walking in the light). And to do this, you must walk in the light. To have fellowship with God and “us”:
Don’t walk in darkness, v.6. And don’t lie to yourself about it. You know what is getting in the way between you and the Lord. Often for teens it’s a relationship. Sin in your life will get between you and the light. The worst place for a Christian to be is in darkness.
Walk in the light, v.6. God is light, Jesus is light, the word is light, the believers in fellowship with him are in light, 1 Thes 5:5. So, get as close as you can to the Lord and his words. His light needs to be very bright in your life. Christians walking in darkness will turn away from the brightness of the light, until they get right with God.
Do the truth, v.6. Obey God’s words. Listen to what God tells you to do in preaching and in reading the Bible, and then do it. Be ye doers of the word, and not hearers only.
Stay clean, v.7-10. We all have sin in our lives. You’re deceiving yourself if you don’t believe this, or you’re calling God a liar. Find out what’s not clean in your life, confess it to the Lord, and then let the blood of Jesus Christ cleanse you from that sin. Christians who start letting sin get between them and the Lord will generally separate themselves from fellowship with believers in a good local church. And their fellowship with the Lord will wane, as well. Don’t let this happen to you. Stay clean.
Conclusion: John wrote these things to you, “that your joy may be full”, v.4. You will experience the fulness of the joy of the Spirit when you do what John wrote. There is no happiness in this world to compare with the joy of walking in the light with God and fellow believers who have come out of darkness and are now living clean.