What does 1 John 1:4 mean?

This verse concludes the introduction and partially explains the purpose of John's letter. The main point of the letter is found in 1 John 5:13.

John wrote to fulfill mutual joy between himself and his readers. Many false teachers opposed the truth of Christ in the early church. Persecutions from the government and the surrounding culture were thrown at those preaching the truth as well. John's letter serves to counter these concerns and build up his fellow believers in Jesus.

John shares similar thoughts in his other letters. For example, 2 John 1:12 says John desired "to come to you and talk face to face, so that our joy may be complete." Third John 1:4 says, "I have no greater joy than to hear that my children are walking in the truth." John's Gospel mentions joy more than seven times (John 3:29; 15:11; 16:20, 21, 22, 24; 17:13). Four of these are in John chapter 16, where Jesus concludes that "your joy may be full" (John 16:24). First Peter 1:8 says, "Though you do not now see him, you believe in him and rejoice with joy that is inexpressible and filled with glory."

Joy is part of the fruit of the Spirit (Galatians 5:22–23) and an expected aspect of the Christian life. Part of John's purpose is showing Christians how to experience joy and fulfillment in their walk.