2 John and 3 John
As an elder in the church of Jesus Christ, the Apostle John knew a special indescribable joy: seeing those he had reared in the faith walking steadfastly in the truth. He writes to a sister in Christ, "I rejoiced greatly to find some of your children walking in the truth" (2 John 1:4). When it comes to those John has personally discipled, he writes, "I have no greater joy than to hear that my children are walking in the truth" (3 John 1:4).
I believe this is a reflection of the heart of God toward all His people. There is no greater joy God has toward us than that we continue faithfully living by the Truth. Sadly, not everyone does this. Some continue mentally agreeing with the truth but don't live it out, relegating their Christian life to merely true talk, but lacking a true walk. Others begin with the truth of the Gospel but then drift off into side roads of new ideas, always learning but never fully coming to the Truth. Others become like Diotrephes (3 John 1:9), holding to Christian teaching, but rejecting the authority of those God has put in place in His kingdom. This results in a life of spitting out many truths from God's Word, while spitting on the very ambassadors of Jesus.
This charge stands true for us today: "Watch yourselves, so that you may not lose what we have worked for, but may win a full reward" (2 John 1:8). It is possible, as a Christian, to lose in eternity all or much of what we work for in this life. (For confirmation, see 1 Corinthians 3:13-15). The reason has to do with whether we are walking daily in truth. Walking is the step by step trajectory of our lives. You and I are walking today, step by step. How will we choose to walk? "Everyone who goes on ahead and does not abide in the teaching of Christ does not have God" (2 John 1:9). The aim is never to go off and find some new thing. Perhaps we need to remind ourselves of the old things of God's Word, go deeper into them, and let Him refresh us and change us as we do so.