That your joy may be full — So our Lord also, John 15:11; John 16:22. There is a joy of hope, a joy of faith, and a joy of love. Here the joy of faith is directly intended. It is a concise expression. Your joy — That is, your faith and the joy arising from it: but it likewise implies the joy of hope and love.
Related Commentaries of 1 John 1:4
1 John 1:4
4 And these things write we unto you, that your joy may be full.